An introduction to astronomy: designed as a text-book for the use of students in college. By Denison Olmsted ...

THE CALENDAR. 49
times as large as a degree on a circle of I foot. A degree
therefore of the limb of such an instrument would occupy a
space of 2 inches; one minute, -' of an inch; and one second,
lr W of an inch.
1 18. But the actual divisions on the limb of an astronomical instrument never extend to seconds: in the smaller instruments they reach only to 10', and on the largest rarely lower
than 1'. The subdivision of these spaces is carried on by
means of the Vernier, which may be thus defined:
A VERNIER is a contrivance attached to the graduated limb
of an instrument, for the purpose of measuring aliquot.parts
of the smallest spaces into wh/ich the instrument is divided.
The vernier is usually a narrow zone of metal, which is
made to slide on the graduated limb. Its divisions correspond
to those on the limb, except that they are a little larger,* onetenth, for example, so that ten divisions on the vernier would
equal eleven on the limb. Suppose now that our instrument
is graduated to degrees only, but the altitude of a certain star
is found to be 40~ and a fraction, or 40~+x. In order to
estimate the amount of this fraction, we bring the zero-point
of the vernier to coincide with the point which indicates the
exact altitude, or 40~+-z. We then look along the vernier
until we find where one of its divisions coincides with one of
the divisions of the limb. Let this be at the fourth division of
the vernier. In four divisions, therefore, the vernier has
gained upon the divisions of the limb, a space equal to x; and
since, in the case supposed, it gains -- of a degree, or 6' at
each division, the entire gain is 24', and the arc in question is
400 24'.
1 19. As the vernier is used in the barometer, where its
application is more easily seen than in astronomical instruments, while the principle is the same in both cases, let us see
how it is applied to nmeasure the exact height of a column of
mercury. Let AB (Fig. 13) represent the upper part of a
barometer, the level of the mercury being at C, namely, at 30.3
inches, and nearly another tenth. The vernier being brought
In the mnore modern instruments the divisions of the vernier are smaller
than those of the limb.
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