1st. The line AB (the distance to be determined) is extended upon the bank to D, from which point, after having marked it, lay off equal distances DC and Cd; produce BC to b, making Cb = CB; then extend the line db until it intersects the prolongation of the line CA at a. The distance ab is equal to AB or the width of the river.
2d. Lay off any convenient distance, BC, perpendicular to AB, erect a perpendicular DC to AC, note the point D where it intersects AB produced; measure BD; then
| AB = | BC ² | . [3] |
| BD |
37. How can the breadth of a river be ascertained by the means of the peak of a cap, or cocked hat?
158]
Place yourself at the edge of one bank, and lower the peak of the cap, or point of the hat till the edge cut the other bank, then steady your head, by placing your hand under your chin, and turn gently around to some level spot of ground on your own side of the river, and observe where your peak or point of your hat again meets the ground; measure this distance, which will be nearly the breadth of the river.
38. How do you ascertain the distance of an object by means of the tangent scale of a gun, the height of the object at the required distance being known?
Direct the line of metal of the gun on the top of the object; then raise the tangent slide till the top of it and 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.
39. What composition may be used for greasing the axle-trees of artillery carriages?
Hog’s lard softened by working it. If this cannot be procured, tallow or other grease may be used; if hard, it should be melted with fish-oil.