Plane trigonometry, by S.L. Loney.

CHAPTER III.

SIMPLE PROBLEMS IN HEIGHTS AND DISTANCES.
41. ONE of the objects of Trigonometry is to find the
distances between points, or the heights of objects,
without actually measuring these distances or these
heights.
42. Suppose 0 and P to be two points, P being at a
higher level than 0.
Let 0/' be a horizontal line N -- P
drawn through 0 to meet in M
the vertical line drawn through
P.
The angle MOP is called o rM
the Angle of Elevation of
the point P as seen from 0.
Draw PN parallel to MO, so that PN is the horizontal line passing through P. The angle NPO is the
Angle of Depression of the point 0 as seen from P.
43. Two of the instruments used in practical work are the Theodolite and the Sextant.
The Theodolite is used to measure angles in a vertical plane.
The Theodolite, in its simple form, consists of a telescope attached
to a flat piece of wood. This piece of wood is supported by three legs
and can be arranged so as to be accurately horizontal.