Lecture 2
On Astronomical Instruments
Sextant
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for the lecture taken from the Royal Institution's archives:-
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Book Volume 1 f139
Thursday
March 2nd 2 o'clock Mr Babbage Astronomy
Lecture
II.
The
difficulties peculiar to Astronomy, and the means of obviating them. Plan to be
pursued. Description of Instruments. The Pendulum the Measure of Time; its
irregularities; the means of correcting them. Astronomical Quadrant and
Circular Instruments; Hadley's Quadrant; Artificial Horizon; Transit
Instruments.
[Pages 1
- 18 incl. of the manuscript for the lecture are missing]
... other instrument, and as our imaginary
astronomer will have occasion to make use of it, some description may not be
uninteresting. The principle on which it depends is the apparent coincidences
of two images: one seen by reflexions,
the other viewed directly. For this purpose there are two mirrors. One of these
has a part of the quicksilver scraped off; through this vacant part one of the
objects is viewed directly, and the other object is reflected from the largest
mirror to that part of the small one, which is silvered, the index being moved
until they coincide. For the purposes of more accurate observation a small telescope
magnifying a few times is usually added. By this means the objects become
better defined, and the results are more to be relied on.
The great
circumstance on which the vast utility of this instrument depends is this. That
during an observation it may be held in the hand. There is no necessity for any
steadiness any firm support. It is this which makes it valuable to the
navigator admidst the unsteadiness and fluctuations of the element which he
traverses. On this he securely relies.
Notwithstanding the motions to which
the ship is necessarily subjected, he can make the most accurate observations
when he has once, by means of these glasses, brought the edge of the Sun to an
apparent coincidence with the distant horizon. It will appear immoveably fixed
whatever may be the agitations of the vessel on which he is placed. It is this
stability which is so desireable. It enables him readily to measure their
angular distance, an object he could have effected by no other instrument. He
has no occasion either for a spirit level or a plummet; both would be useless
in such a situation. And no other instrument can be rectified without them.
There is
another advantage peculiar to the sextant. It is the only instrument by which
the distance of two celestial objects can be immediately ascertained without
the trouble of any calculation. This circumstance renders it of frequent use in
ascertaining the distance of the Sun from the Moon, or of the Moon from a star.
These means are constantly employed by seamen for determining the longitude.
When this instrument is used to
ascertain the altitude of an object at sea, it is made, by reflections from the
mirror, to coincide with the horizon, which is always at the same distance if
the observer is at the same altitude above the level of the water. But when an
observation is to be made on shore, the case is altered. There is no horizon to
which the object can be compared. At least there is no certain and regular one.
The termination of our prospect is generally broken and uneven, intercepted by
trees or by hills. From this cause it would, at first sight, appear that the
sextant would be useless in observations on the altitude of the heavenly bodies
when made on land. There is however, a method by which this inconvenience experienced
by the want of an accurate horizon may be obviated. It was observed that, when
we look at a smooth piece of water, all the objects around appear represented
in it, but in an inverted position. The houses or trees which are adjacent to
it are reflected in the water of exactly the same magnitude the objects really
appear to the eye. In fact, the water acts just in the same manner as a looking
glass would if laid perfectly horizontal. If now the Sun is above the horizon,
it will be reflected in the water and its apparent depression below the surface
will be exactly equal to its real elevation. If therefore, we have any means of
measuring the angular distance of the Sun from its reflected image in the
water, it is clear that its real altitude above the surface of the water must
be equal to half this measured angular distance. The reason is evident. The
Sun's image appears just as much below the surface of the water as it itself is
actually above that surface.
Now by
means of the sextant this distance may be easily measured. The image of the Sun
in the water may be viewed directly and the reflected image may be made to
coincide with it. This was doubtless the object which first suggested the idea
of an artificial horizon, but it was soon found that the surface of the water
exposed to the active air was too unsteady to admit of much accuracy in the
observations: various others were substituted, but that which is now
universally adopted is quicksilver.
This brilliant fluid reflects more
light than any other we are acquainted with and is on that account admirably
adapted to the purpose. The instrument in which it is employed, is of very
simple construction, being nothing more than a small wooden tray filled with
mercury. This is covered by a kind of roof formed of two bits of plate glass;
the use of this roof is merely to protect the quicksilver from the undulations
which might arise from the action of the air on its surface. There are but few
cautions to be observed either in its construction or use. It is only necessary
that the two surfaces of each glass ...
[remainder
of lecture missing]
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| Artificial Horizon |
