Place a vessel of water upon the ground and recede from it until you see the top of the object reflected from the surface of the water. Then it will be:—As your horizontal distance from the point of reflection is to the height of your eye above the reflecting surface, so is the horizontal distance of the foot of the object from the vessel to its altitude above the said surface.
Instrumentally.
Read the vertical angle, and multiply its natural tangent by the distance between instrument and foot of object; the result is the height.
When much accuracy is not required vertical angles can be measured by means of a quadrant of simple construction, represented in [Fig. 46]. The arc AB is a quadrant, graduated in degrees from B to A; C, the point from which the plummet P is suspended, being the centre of the quadrant.
Fig. 46.
When the sights AC are directed towards any object, S, the degrees in the arc, BP, are the measure of the angle of elevation, SAD, of the object.
TO FIND THE DEPTH OF A SHAFT.
Rule:—Square the number of seconds a stone takes to reach the bottom and multiply by 16.