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

50 THE EARTH.
(by a screw which is usually attached to it) to coincide with the
surface of the mercury, we look along Fig. 13.
down the scale, until we find that the
coincidence is at the 8th division of A
the vernier. Now as the vernier gains
y-lo of o-l lo of an inch at each division upward, it of course gains ry in C
eight divisions. The fractional quan- 1
tity, therefore, is.08 of an inch, and I
the height of the mercury is 30.38. If l
the divisions of the vernier were such,
that each gained - (when 60 on the
vernier would equal 61 on the limb)
on a limb divided into degrees, we
could at once take off minutes; and
were the limb graduated to minutes, BlI
we could in a similar way read off seconds.
1 20. The instruments most used for astronomical observations, are the Transit Instrument, the Astronomical Clock, the
Mural Circle, and the Sextant. A large portion of all the
observations made in an astronomical observatory, are taken
on the qneridian. When a heavenly body is on the meridian,
being at its highest point above the horizon, it is then least
affected by refraction and parallax; its zenith distance (from
which its altitude and declination are easily derived) is readily
estimated; and its right ascension may be very conveniently
and accurately determined by means of the astronomical clock.
Having the right ascension and declination of a heavenly
body, various other particulars respecting its position may be
found, as we shall see hereafter, by the aid of spherical trigonometry. Let us then first turn our attention to the instruments employed for determining the right ascension and decli.
nation. They are the Transit Instrument, the Astronomical
Clock, and the Mural Circle.
121. The Transit Instrument is a telescope, which is fixed
permanently in the meridian, and moves only in that plane.
It rests on a horizontal axis, which consists of two hollow
cones applied base to base, a form uniting lightness and