Monday, 11 November 2013

Lecture 6: On the Sun

Lecture 6




On the Sun

            All the heavenly bodies participate in the apparent motion which arises from the diurnal rotation of the Earth, but several of them possess a proper motion of their own. The quickness of this change of place is most evident in the Moon, a body which we have already considered. It is these proper motions which it is most interesting to follow, because it is from them alone we can derive a complete knowledge of the systems of the world. In discovering the distance of a terrestrial object we observe it in two different positions. The same principle must be applied to the heavens.

            In our attempts at discovering the laws of Nature we must observe her in various points of view and detect these laws from the change of appearance she presents to us. There is, however, this striking difference between the philosophical enquiries of Astronomy and those of any other science. In the various branches of natural philosophy we may vary the phenomena presented to us by means of experiments and thus put our theories to the test; but the case is widely different in Astronomy: Nature presents the phenomena and the skill and industry of Man must be directed to search for those situations from which he may most easily discover the chain of cause by which she operates.

Of all the heavenly bodies which appear to have a proper motion of their own the most brilliant and remarkable is the Sun. Its proper motion is in a direction contrary to the daily motion of the Earth. This will be evident from the appearance of the heavens during the night which changes and is renewed with the seasons.

            The stars which are situated in the path of the Sun and which set a little time after him soon are lost in his light, and at length reappear before his rising. The Sun therefore advances towards them in a direction contrary to his diurnal motion. It was by this means that his proper motion was examined by the ancients, but at present it is determined with much more accuracy by observing every day his altitude on the meridian and the time which elapses between his passage and that of some known stars across the meridian.

            By these means it has been found that the Sun appears to move in an orbit round the Earth, that this orbit is not in the plane of the equator but inclined to it at an angle of about 23 degrees 28 minutes. The path in which the Sun moves is called the ecliptic. This path intersects the equator in two points; these are named the equinoxes. The reason is that, when the Sun is situated in either of them, it appears by the daily motion of the Earth to move in the equator. And, as the horizon of every place divides the equator into two equal parts, the day will then be equal to the night in every part of the Earth. The Sun now advances from the equinox of Spring and its altitude on the meridian increases daily. The arc it describes continually augments and increases the length of day, until it arrives at its greatest altitude. At this period of year the days are longest, and the situation at which the Sun has arrived is named the summer solstice. The reason of this name is that during several days the Sun preserves nearly the same meridianal altitude and seems therefore in a manner stationary. The circle which the Sun appears to describe is called the tropic of summer. There is a similar one represented on the globe.

It now redescends to the equator and continuing its course it arrives at the winter solstice, where its altitude is least. This is the shortest day of the year. When the Sun has arrived at this term, it again ascends and returns to the vernal equinox. Such is his constant progress and that of the seasons. The spring is comprised between the vernal equinox and that point of the ecliptic where the sun appears stationary. The interval between this point and the autumnal equinox forms the summer. Autumn is the period which elapses between this equinox and the winter solstice, and the remaining part of the year, between the winter solstice and the vernal equinox, constitutes winter.

            As the presence of the Sun above the horizon causes heat, it might perhaps be imagined that the temperature would be the same during the spring as in the summer, and in autumn the same as in winter. But temperature is not the instantaneous effect of the presence of the Sun, it is the consequence of its continued action. During the course of a day it does not produce the greatest effect until a considerable time after it has arrived at it highest altitude. The different degrees of heat in summer and in winter do not then arise entirely from the different times the Sun is above the horizon. The direction and force of the solar rays have a considerable influence.

During the short days the effect of the Sun is less both with respect to the intensity of the Sun's rays, their direction and the time of their continuance; and during the long days it is greater in all these respects. It may also be remarked that the severest frosts usually take place after the days have begun to lengthen, and the most oppressive heat prevails when the days are decreasing: the reason of which is, that during the summer months the Earth, having imbibed more heat than it gave out, is not exhausted of its superabundant warmth until towards the close of the year. In a similar manner the waste of the Earth's heat being greater in winter than its supply, it continues to get colder and colder until a month or longer after the winter solstice. From a similar cause arises the difference between spring and autumn though the position of the Earth is the same in both.

That remarkable variety of seasons which takes place in the various parts of the world is owing to the different elevation of the pole in the different climates. To a spectator situated on the equator the poles are on the horizon. The day is consequently always equal to the night and at the two equinoxes the Sun passes through the zenith of the place, and the least meridian altitude of the Sun happens in the solstices. In this situation a curious circumstance takes place. On the 21 June and 21 December the shadow of an upright rod will fall in two opposite directions. On one of those days it will be directed toward the north on the other to the south point.

            This is a circumstance which can never take place in our climate. At noon the shadow will always be directed to the north. The inhabitants of the equator enjoy two summers and two winters every year. And the same happens to all those countries where the height of the pole is less than the obliquity of the ecliptic. Beyond this limit there is only one summer and one winter in every year. As we approach the pole the longest day in summer augments and the shortest in winter decreases. To the inhabitants at the polar circle on the 21 June, which is the summer solstice, the Sun never sets and similarly, on the day of the summer solstice, it never rises. As we approach still nearer the poles the continuance of the Sun above the horizon becomes longer it exceeds several days and even weeks. At the pole itself it rises and continues to illuminate it for six months. It then sets and is invisible for an equal length of time.

By observations on the Sun, that is by comparing its place with that of known stars, it is found that it moves faster in winter than in the summer months. This variation in its motion is unequal. As in the case of the Moon we found that the apparent diameter is variable. It is natural to examine whether the same does not take place with respect to the Sun. On applying a micrometer to this luminary we find that it appears larger in winter than in summer, that is, when the velocity of the Sun is greatest, its diameter is largest and conversely, when the Sun moves slowest, its diameter is least. From this we immediately infer that the Sun is at a greater distance from our Earth in the summer when she moves slowest than she is during the winter months when her apparent diameter is greatest.

            The apparent diameter of the Sun only informs us by its changes of the relative variation of our distance from that body. What may be our real distance becomes an object of considerable interest. The methods of ascertaining this have been successively improved, but the extreme accuracy with which the observation ought to be made and the minuteness of the quantities to be determined will probably, for a long time, render the result in a measure uncertain. The easiest method of finding our distance from the Sun is by employing a method similar to that by which we measure distances on the Earth.

It has been shown that, if at the two ends of a line of known length we measure the angle formed by a distant object, we may, by calculation, determine the distance of this latter object. To apply this to our case let us suppose two observers, one situated in the northern hemisphere the other on the same meridian on the southern. If, at the same instant, they observe the altitude of the Sun's centre, it is obvious we shall have the two angles required. From these observations we may determine the angle under which the Earth's diameter would appear to a spectator at the Sun. It is this angle which is called parallax, but it is too small a quantity to be determined by this method. All that we can learn from it is a negative result. We may infer that the Sun cannot be nearer to the Earth than 6,000 of its diameters, but how much farther off that luminary [we] may be situated we cannot from this method determine.

            Astronomy, however, affords other means of ascertaining the Sun's parallax which will be hereafter noticed. The most successful is the transit of an inferior planet over the disc of the Sun. From this it would result that the Earth is situated at rather less than 95,000,000 miles distant from the Sun. This is determined from the Sun's horizontal parallax, which is very nearly 8 seconds [of arc], and we may form some idea of the investigation when it is considered that an error of 1 second in this small quantity would produce an error of many millions of miles in our estimation of the Sun's distance.

On the surface of the Sun black spots are frequently observed. Their number, position and magnitude are variable. They are sometimes very numerous and of considerable extent. Some have been observed which have been four or five times large as the Earth's diameter. Sometimes, though rarely, they have disappeared altogether for a few years. The solar spots are almost always surrounded by a penumbra, which is enclosed in a luminous cloud, which is almost always more brilliant than the rest of the Sun's body. Whatever may be the nature of these spots it is sufficient for our present purpose that they preserve their relative situation on the Sun's surface, and though variable they continue fixed for a few months or perhaps years.

            It is by means of these spots that the astronomer ascertains that the Sun, like the Earth and the Moon, revolves on its axis. This immense body, which is at least 1,300,000 times larger [in volume] than the Earth, turns on its axis. From accurate observations on the returns of the spots the time which this revolution occupies is about 25 days and a half. This revolution might appear slow, but when we take into consideration the immense magnitude of the Sun it will be found that its equatorial parts are moving at the rate of 5,000 miles in an hour. The axis round which the Sun turns is not exactly perpendicular to the plane in which the Earth moves. It is inclined at a small angle, about 7  degrees.

There is a curious remark relative to the solar spots already alluded to. They are almost all contained in a zone which extends about 30 degrees on each side of the equator. It is but very rarely that they have reached as far as 40 degrees.

            The intensity of the Sun's light is not the same on every part of his disc. It ought according to theory to be much brighter towards the border than in the centre of the Sun, but the reverse is the fact. This has been shown by the experiments of Bouguer. He admitted rays of light from the Sun into a darkened room, but, as their light would be still too intense he diminished it, by causing it to pass through a concave lens. It now became weakened and could readily be compared to the light of a wax taper. He repeated these experiments with rays from various points of the Sun's disc and compared them with the light of the Moon. He came to the conclusions that the light of the Sun is 300,000 times as great as that of the Moon, that some parts of the Moon's disc are 3 times as bright as others and that the centre of the Sun is considerably more brilliant than the edge. The light of the Sun's border must therefore be obscured or extinguished in a certain degree and it becomes interesting to enquire into the cause. The only one which affords a complete explanation is that the Sun is surrounded by a very dense atmosphere; the rays which issue from his limb are obliged to traverse a more considerable portion of this atmosphere than those which emanate from his centre.

It appears from this that there is a very great probability of the Sun's possessing an atmosphere. There is indeed another phenomenon which has been explained  on the hypothesis of its existence and which would assign to it very considerable limits. I refer to that faint light which is sometimes visible a little before the rising and after the setting of the Sun, particularly in the spring and which is called the zodiacal light. It appears generally at the end of winter and in the beginning of spring after sunset and it appears before sunrise in autumn and at the commencement of winter. It resembles in form a pyramid lying lengthways along the zodiac, its base being placed obliquely in respect to the horizon. This phenomenon was first noticed by Cassini the Elder, in 1683. The zodiacal light is, according to Mairan, the solar atmosphere. It is a rare and subtile fluid, either luminous itself or made so by the rays of the Sun. He supposes it to surround the Sun but in greater quantity and more extensively about his equator than at any other part.

The length of the zodiacal light varies, sometimes in reality sometimes only in appearance. Its length is seldom less than 60 degrees nor its breadth more than 20, but it has been observed to extend to 100 or 103 degrees, and then its breadth was only 8 or 9 [degrees]. It has, however, been suggested that this phenomenon is not owing to the atmosphere of the Sun, but that this body is the fountain of electricity, which  is thrown off from the equatorial parts from the rapidity of its rotation on its axis, and, it is added, as a probable conjecture, that the zodiacal light, the tails of comets, as well as the aurora borealis and lightning   are only its various and not dissimilar modifications.

            The succession of our own ideas or of external objects present to us the notion of time. We conceive time, or absolute time, to flow uniformly in an unchangeable course. This alone measures the changing of all other things and unless we apply to the common measures of time, which are gross and inaccurate, some proper corrections, the conclusions we arrive at are found to be erroneous. It becomes then necessary to fix on some natural object whose motion is conspicuous and nearly uniform, or some artificial contrivance to determine this measure. Of the first kind the Sun has been chosen and of the latter, a pendulum clock, but as the motion of the Sun is not uniform and as a clock is liable to variations from heat and cold and other accidental causes, it becomes desirable to establish some mode of comparison by which we may gain a correct measure of time. This is best effected by tracing out the irregularities of the Sun's motion.

The astronomical day at any place begins when the Sun's centre is on the meridian of that place. It is divided into 24 hours, which are reckoned from one to 24. The interval of time between two successive transits of the Sun over the same meridian is called a solar or astronomical day.

            At the end of a diurnal revolution of the Earth which is known to be a uniform motion, the same star will come to the meridian that transited it at the preceding noon, but the Sun during this period has moved from that star to another which has greater Right Ascension. Therefore, before the Sun can be again on the meridian, the Earth must have described this additional arc. This may, perhaps, be illustrated by considering the hands of a clock. At midday the two hands are both together and point to the hour of twelve. After one revolution of the minute hand they are not again together for the hour hand will have advanced about 5 minutes and will point to one. The minute hand must therefore move over more than a whole circle before the two are in the same relative position. The apparent motion of the Sun is unequal in different parts of the year. A dial therefore which measures time by the Sun's motion will measure it unequally. Hence there ought to be a difference between the hour indicated by a sun dial and that by a good clock. This difference is called the equation of time. The two principal causes of this equation of time are the inclination of the Earth's orbit to the equator. This is generally called the inclination of the ecliptic. And the other cause is the unequal motion of the Sun in its orbit.

We have hitherto only considered this question on the hypothesis of the Sun revolving round the Earth, but all the appearances we have described are equally capable of explanation by supposing the Sun at rest and the Earth revolving round it. It remains then to enquire which is the fact in Nature. A similar reason has already occurred with respect to the question whether the heavens revolve around the Earth at rest or whether this latter body moves on its axis in the contrary direction. The appearances are the same in both cases, but we have decided both from reason and experiment that the latter is the true theory.

            In the case of the annual motion of the Earth we must depend on arriving at a solution rather from reason and analogy than from any direct experiment. The following are the considerations which lead us to adopt this conclusion. The mass of the Sun is immensely greater than that of the Earth. Is it not therefore more simple to make the latter body revolve round the Sun than to put the whole solar system in motion round the Earth?

Besides this latter supposition involves a physical impossibility. It is known from the laws to which matter is subject that, when two bodies acting on each other revolve round their common centre of gravity, it is physically impossible that the larger can revolve round the smaller one at rest. This is precisely the case with the Sun and in physical point of view decides the question, but it may nevertheless be useful to state a few of the other arguments by which this important point is supported. The analogy of the Earth compared with other planets strongly confirms it. They all revolve on their axis and several have satellites or moons. An observer situated on any one of them would have just as much and, in some cases, a greater right to consider himself the centre of the system, and to view the Sun as a dependent body revolving round his world for the purpose of affording him light. Shall we then presume that what would be an illusion in each of these worlds would be a fact in ours? There is no apparent difference to justify such an inference. These bodies, like our own globe, shine with a borrowed light. The Sun alone is the source of light, the largest body in the system. Shall we then refuse to consider him in its centre? If we conceive ourselves for a moment transported to the Sun, this arrangement will restore order and harmony to the system. The annual motion of the Earth will form no exception to the general laws of Nature.

I shall only state one other fact which clearly proves the point in question. This arises from the progressive motion of light. About the close of the 17th century Roemer discovered that the propagation of light is not instantaneous, but that it takes about 8 minutes in passing from the Earth to the Sun. In consequence of this when we observe an eclipse of Jupiter's satellites, if this planet is in opposition, it will appear to begin sooner, and, if he is in conjunction, it will seem to commence later than it ought by calculation. This and many other phenomena are all explained by supposing the Earth in motion round the Sun. It is therefore strong presumptive evidence of this fact. It appears then that the Earth revolves around the Sun once in about 365 days and on its own axis once in 24 hours. The knowledge of these important truths were not the result of the labours of one individual. Their perfect demonstration was the united work of ages.

Man ignorant of the true constitution of the Universe and seduced by self-love and by the illusion of his senses for a long period considered himself as placed in the centre of the movements of the heavenly bodies, and his pride was sufficiently punished by the vain terrors with which they inspired him. In throwing aside the veil, which concealed from him the true system of the world, he perceived himself far from the centre of the Universe, placed on a planet almost imperceptible in the vast extent of the Solar System, which itself is but a point in the immensity of space. But the sublime knowledge at which he has arrived concerning these grand and interesting objects may well console him for the scanty limits he occupies in the scale of Nature.

            We have already observed that that elevation of water, called a tide, is caused by the attraction of the Moon acting on the water of the ocean, and that this is augmented and diminished at different times by the action of the Sun. The atmosphere which surrounds our globe being a fluid body might be supposed to be acted upon by the same influence, and the consequence would be winds and variations in the height of the barometer, which would recur at the same intervals as the tides do in the sea. This we should expect from theory, which at the same time informs us that the variations of the barometer, even in the most favourable situations, such as at the equator, must be very small. The greatest is calculated at ?th part of an inch [of Hg]. It is evident that this very minute quantity cannot always be observed when the variations from other causes may produce a difference of several inches. The Sun then, on account of its attraction, has but a small effect in producing winds. On another account, namely by the heat which it affords, its effect is much more considerable.

Several philosophers, amongst whom we meet with the name of Descartes, have ascribed the general winds to the diurnal motion of the Earth. They contend that, as the Earth turns eastward, the particles of air being very light are left behind, so that, in respect of the Earth's surface, they move in a contrary direction and thus become an easterly wind, which is generally the case near the equator. It is sufficient for the refutation of this hypothesis to observe that it contradicts the laws of mechanics, and that the air being attracted to the Earth by gravity would, in a short time, acquire the same velocity.

            Dr. Halley, dissatisfied with this explanation, substituted another cause capable of producing the same effect and more consistent with the known laws of the motion of fluid bodies. He attributes the trade winds to the action of the Sun's rays heating the air. Thus the air being rarefied by heat must become lighter and ascend, and fresh air must rush in to supply its place and bring the whole to an equilibrium. But from the motion of the Sun an equilibrium cannot take place. There will therefore be a constant current produced. By combining this cause with the variations which must arise from the situation of continents and chains of mountains Dr. Halley explained many of the regular winds which blow periodically.

From the action of the Sun the air will be heated and consequently ascend. There will therefore ensue two currents in the upper regions of the atmosphere. There will be a stream from the equator to the poles, and in the lower strata near the Earth's surface there will be a contrary current from the pole to the equator. This combined with the motion of the Earth on its axis will produce a wind from west to east, which is in fact the direction of the trade winds.

            The density of the Sun, which is about  part of that of the Earth, indicates that this body consists of something more solid than a simple flame. If we admit the ideas of Buffon concerning the formation of the planets, we should consider it a body exactly similar to our Earth, only in a state of fusion. This celebrated naturalist has explained at considerable length an hypothesis, according to which the planets primitively constituted a part of the Sun. According to this theory a comet moving with immense velocity grazed along the surface of the Sun and tore from it and projected to a distance some portions of its body. These being in a state of fusion by the mutual attraction of their particles formed themselves into globes which continued to circulate round the Sun, from the combined effect of their centripetal and centrifugal forces. Some detached parts of the largest masses formed the satellites, which circulate round them. These bodies gradually parted with the heat that kept them melted. The smallest bodies cooled soonest and the larger masses more slowly. Buffon even attempted to calculate the time they would require to cool, by comparing it with that in which known weights of red-hot iron required for the same purpose. He found that the Moon and other satellites were already cooled below the temperature of ice.

That the Earth must have required 75,000 years to arrive at her present temperature, and that she would continue insensibly to lose her heat, and that, at the end of about 93,000 years more, the zones of ice at the poles would increase to the equator and thus put an end to the human species. Such is the ingenious fiction of this celebrated naturalist. It is more to the credit of his imagination than to his acquaintance with the mechanical philosophy. It will be sufficient to mention one insurmountable objection to this theory. It is known from mechanical principles that, if this had been the origin of the planets, they must all have returned nearly to the Sun's surface once in each revolution. But this is far from the case: they move in ellipses of very small eccentricity.

            The phenomenon of the solar spots are a subject on which astronomers have entertained various opinions. In considering the Sun as an immense ocean of melted fluid it was natural to regard the spots as the scoria floating on the matter in fusion. But it may be observed that we have no proof of the Sun's being a body in such a state. Granting however that it were what causes the formation of new scoria. For this purpose we must suppose new bodies to have fallen onto the Sun, for we well know that, when a pot of melted metal has been sufficiently purified by fusion, no new scoria arises unless from the accession of fresh matter which finds a difficulty in uniting with the former. Perhaps, however, it may be said that some comet has fallen into the Sun and caused these spots, but this is a mere conjecture and can but rarely happen, besides the cause, if it ever happens, would not answer the effect.

Another theory which has met with several supporters considers the Sun as a solid nucleus covered with a melted fluid, that from certain causes this fluid is subject to several motions of flux and reflux, and that in consequence of these motions the mountains and other asperities with which this solid nucleus is covered are sometimes left exposed by the recess of the fluid, and at other times covered by its return. This will account for the successive appearance and disappearance of spots in precisely the same place, but this system, though more probable than the former, is founded on arbitrary suppositions and is contrary to observation.

            In fact, if a solar spot were a mountain of the solid nucleus elevated above the melted matter, it would, when it arrives near the edge of the Sun, form a new kind of dark projection. This, however, very rarely happens. On the contrary a spot, as it approaches the Sun's border, becomes narrower and at length disappears. This clearly indicates that it is not an elevation above the Sun's surface.

Lalande made a great many observations on the solar spots and concludes that they result from the scoria attaching themselves to the summits of the mountains. This was published in the Memoirs of the French Academy in the years 1776 and 1778. Nearly at the same time appeared in the Philosophical Transactions a very different hypothesis. In this the Sun was regarded as a solid nucleus covered with a semi-fluid ignited substance. The cause of the appearance of the spots we supposed to be owing to the effects of volcanoes, which by their action drove aside the semi-fluid matter and exposed to view the dark nucleus. After the eruption the half-melted matter gradually subsided into the vacant space and thus the spot disappeared until a new eruption should again produce the same effect.

            There is, in this theory of the cause of the spots on the Sun's disc, an appearance of the truth, which renders it inviting. It would be still more so if the solar spots always presented the same appearances. Its ingenious author was, however, aware of many of the objections that might be urged against it and endeavoured in a subsequent paper to explain them.

Such is an account of the chief of those theories of the Sun which consider this body in a state of violent ignition. It may be remarked that as yet we are scarcely acquainted with the nature either of light, of fire or of heat, nor even with all the means by which they are produced. They are affections of our senses, and it may not be necessary that the Sun or other object which produces them should itself be either light or fire.

            As a system which supposes light to be emitted from the Sun, notwithstanding the difficulties which it presents, appears to meet with the largest number of advocates, there results from it a curious question relative to this body which we shall now examine. As the Sun is the source of a continual torrent of luminous particles, how does it happen that during so many ages it has not been entirely exhausted? During 2,000 years we possess astronomical observations, and these indicate no diminution either in mass or volume. Comets which have frequently destroyed the finely woven web of the theorist have, on this occasion, been invoked to supply the imaginary waste of matter in the Sun, but a more probable explanation may be given without their aid. It may be observed that many millions of years would have but a small effect in diminishing the solar bulk on account of the immensity of his volume and the great levity of light. The philosopher, Niewentiit, has calculated that the  of a grain of wax, which is consumed by a taper in one second of time, produces a greater number of particles of light than 1,000 million of Earth's equal to our own could contain grains of sand. This will afford an idea of the extreme tenuity of the molecules of light and convince us that the Sun would require no fresh supply from comets.

It has already been observed that the Sun is surrounded by a light which extends to some distance and is termed the zodiacal light. It has a lenticular shape and forms a kind of atmosphere. If, therefore, our Sun were viewed from any star situated in direction perpendicular to the solar equator, it would appear to an observer placed there like a small luminous point plunged in a round nebulosity, similar to what we observe in several of the stars, and which Dr. Herschel has ascertained not to consist of a multitude of small stars, but to be a space filled with luminous matter. If the same object were viewed from a star situated in the plane of his equator, the Sun would appear like a star plunged in a nebulous atmosphere of a lenticular shape.

            The spots on the Sun's surface have by their periodic return enabled us to ascertain his rotation on this axis. They have also given rise to several theories respecting their nature. We have already seen that some observers have considered them as owing to the eruption of volcanoes, whilst others have regarded them as the scoria of an immense furnace. Neither of these systems accords with the latest observations of Dr. Herschel. In the year 1776 this astronomer discovered a spot on the Sun sufficiently large to be visible to the naked eye. Having observed it with a 7 foot telescope and a very high magnifying power, it appears to him divided into two parts, the largest of which had an extent of 30,000 miles; the total length of the whole spot was about 40,000. This spot evidently occupied too large an extent of surface for a volcanic eruption. During the years 1783, 1791 and 1792 Dr. Herschel saw a great variety of spots on the Sun's disc. Many were much lower than his apparent surface. These spots did not appear like floating masses, but resembled portions of his solid nucleus seen through an atmosphere driven aside by some great agitation.

According to the theory of this admirable observer the surface of the Sun is very unequal: there are numerous deep cavities and lofty elevations. Above the solid body of the Sun he places a very extensive atmosphere composed of elastic fluids, of which some are luminous and others transparent. He compares the formation of the luminous fluids in the solar atmosphere to the formation of the clouds in that of the Earth. He perceives in both bodies two immense laboratories where the different combinations and decompositions are carried on in a manner analogous to the chemical action of the bodies they contain. He estimates the height at which these luminous clouds are formed in the solar atmosphere at not much less than 1,800 miles, nor much more than 2,700.

            In considering the atmosphere of the Sun and the numerous points in which this body resembles the planets in its solidity, in its lofty mountains, its deepest cavities, in its rotation on its axis and in the gravity of the bodies at its surface Dr. Herschel was led to view this star as an immense and brilliant planet, which alone deserves the name of a primary one, and to consider it a habitable globe. This ponderous body ought not to be viewed, according to this able astronomer, merely as a centre of attraction appointed to return the planets in their orbits, but for the nobler purpose of affording an abode adapted to the reception of innumerable generations of living things. The same idea has been extended to the stars, which in many respects resemble our Sun.

It must not be dissembled that this beautiful and interesting theory is subject to a great objection which may be urged on account of the heat, which, according to the impressions we receive from it on the Earth, ought at the Sun's surface to exceed any thing of which we can have a conception. To this objection founded on the nature of our sensations Dr. Herschel replies, that the solar rays may possibly not carry heat along with them, and that that which they excite may depend entirely on the nature of the bodies on which they fall. But can these rays, which at the distance of 95,000,000 miles melt the metals, be entirely without energy at the brilliant body from which they emanate, or shall we suppose that the substances which exist at the Sun's surface are so constructed by Nature as not to receive the strong impression which they produce? These are questions we can hardly hope to solve. Of the theory itself it may be observed that it is more consistent with facts than any which has been proposed and that it accords with the universal provision of Nature by which the material universe every where beams with animated life.

It is from observation alone that we derive materials for the formation of any legitimate theory, but in the case of the object we are now considering some difficulties arise from the powerful light and dazzling brilliancy with which it shines. The human eye is too delicate an instrument to endure for a moment the solar rays: some shade must be made use of to diminish their power and protect it from their influence.

            For common purposes, such as viewing the commencement of an eclipse, or in observing the Sun with a telescope of very small magnifying power, it is sufficient to smoke one of the glasses made use of in the flame of a candle: but when it is necessary to examine the disk of the Sun with telescopes of high magnifying power this contrivance fails.

            With a view to determine the most advantageous method of making observations on the Sun Dr. Herschel instituted a series of experiments which are detailed in the Philosophical Transactions of the year 1800. He found that the smoked glasses were, by the heat of the Sun, soon covered with blisters and endeavoured to substitute glasses differently coloured. The first he made use of was two red glasses. These intercepted sufficient light, but the heat penetrated through them and became insufferable to the eye. Two pale green glasses were used, that next the eye being smoked. This acted incomparably well, but the heat soon passed the first and raised blisters on the smoked side of the second. Many other combinations were tried in which the coloured glasses cracked from heat. The most successful was the application of a very dark green glass with the side nearest the eye a little smoked. This was the arrangement with which Dr. Herschel made a very extensive series of observations on the solar spots, and it always protected the eye very effectively, both from too great light and also from the too powerful effect of heat.

There is, however, another means by which the same purpose may be effected. It is well-known that some substances reflect more heat than others, and that they also reflect different quantities of light. This is remarkable in the instance of glass and the metals. A speculum of a telescope of large aperture will reflect a very considerable portion of heat to its focus. Dr. Herschel therefore, instead of a metal mirror, ground one out of a solid piece of glass, and having made the back of it rough, that it might reflect as little light as possible, he used it in one of his 7 foot telescopes. The effect is that no inconvenience is found from the reflection of heat, and a single coloured glass is sufficient to protect the eye from the Sun's light.