Prev
| Next
| Contents

## TO DETERMINE HEIGHTS OF OBJECTS

/By Shadows/

Set up vertically a stick of known length, and measure the length of
its shadow upon a horizontal or other plane; measure also the length
of the shadow thrown by the object whose height is required. Then it
will be:--As the length of the stick's shadow is to the length of the
stick itself, so is the length of the shadow of the object to the
object's height.

/By Reflection/

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. 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.

/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.

Prev
| Next
| Contents