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

6 THE EARTH.
Table showing the Dp2', of the Horizon at ciffe'ent elevations,
fromn 1 foot to 100 feet.*
Feet. / Feet. / _ Feet. / ~
1 0.59 13 3.33 26 5.01
2 1.24 14 3.41 28 5.13
3 1.42 15 3.49 30 5.23
4 1.58 16 3.56 35 5.49
5 2.12 17 4.03 40 6.14
6 2.25 18 4.11 45 6.36
7 2.36 19 4.17 50 6.58
8 2.47 20 4.24 60 7.37
9 2.57 21 4.31 70 8.14
10 3.07 22 4.37 80 8.48
11 3.16 23 4.43 90 9.20
12 3,.25 24 4.49 100 9.51
Such a table is of use in estimating the altitude of a body
above the horizon, when the instrument (as usually happens) is
more or less elevated above the general level of the earth. For
if it is a star whose altitude above the horizon is required, the
instrument being situated at O (Fig. 1), the inquiry is, how far
the. star is elevated above the level HOR, but the angle taken
is that above the visible horizon OD. The dip, therefore, or
the angle HOD, corresponding to the height of, the point O,
must be subtracted, to obtain the true altitude. On the Peak
of Teneriffe, a mountain 13,000 feet high, Humboldt observed
the surface of the sea to be depressed on all sides nearly 2
degrees. The sun arose to him 12 minutes sooner than to an
inhabitant of the plain; and from the plain, the top of the
mountain appeared enlightened 12 minutes before the rising or
after the setting of the sun.
1 1. The foregoing considerations show that the form of the
earth is spherical; but more exact determinations prove that
the earth, though nearly globular, is not exactly so: its diameter from the north to the south pole is about 26 miles less than
through the equator, giving to the earth the form of an oblate
spheroid,t or a flattened sphere resembling an orange. We; This table includes the allowance for refraction.
if An oblate spheroid is the solid described by the revolution of an ellipse about
its shorter axis.