to ascertain the distance, the height of the object at the required distance being known.
Lay the gun by the line of metal for the top of the object; then raise the tangent scale till the top of it and the notch on the muzzle are in line with the foot of the object, and note what length of scale is required.
Then,—by similar triangles—
As the length of the raised part of the tangent scale
is to the length of the gun;
so is the height of the distant object
to the distance required.
Thus, supposing the height of the object to be 9 feet, the length of that part of the tangent scale which is raised, 3 inches, and of the gun 6 feet, the proportion will be—
As 3 : 72 :: 108 : 2592 inches, or 216 feet.
10.—BY MEANS OF THE PEAK OF A CAP,
to measure the breadth of a river.
Place yourself at the edge of one bank, and lower the peak of your cap till you find the edge of it cut the other bank, then steady your head by placing your hand under your chin, and turn round gently to some level spot of ground on your side of the river, and observe where your eyes, and the edge of the peak again meet the ground; measure the distance, which will be nearly the breadth of the river.
11.—BY THE REPORT OF FIRE-ARMS, TO ASCERTAIN THE DISTANCE
OF ANY OBJECT, vide [Sound], [page 316].