In calculating a height, as that of a tree, house or tower, the triangles can again be used, as shown in diagram B. Choose a level strip of ground; pace the distance in a straight line, from the base of the tree A, or tower, to a point some distance from the tree, and plant a pole or stake say 5 feet high B; continue pacing the straight line to the point where, lying down with eyes level with the tree base, the top of the tree can be seen on a line with the top of the pole; plant here stake C. The height of the tree AA' will be to the length of the distance from C to A as the height of the pole, BB' is to the distance between B and C. A Scout can stand in the place of the stake B. [Illustration: Diagram B. To Measure Height of Tree, Etc.] [Illustration: Diagram C. To Measure Height with a Mirror] There are other ways of determining height. As shown in the diagram C, place a mirror (M) horizontally on the ground reflector side up, some distance from the base of the object to be measured, in this case a tent. Walk backward from the mirror in a straight line until the top of the tent pole can be seen in it. The problem will read in this way: the distance from the mirror to your heels (MS) is to the distance from your heels to your eyes (GS) as the distance from the mirror to the base of the object (MT) is to the height of the object (TT'). Water in a dark pan or tray or a pool on a still day will answer for a mirror. [Illustration: Diagram D. To Test a Right Angle] A right angle can be tested by measuring off 3 feet on one side of the corner and 4 feet on the other side, as shown in diagram d. If the distance between the two points is 5 feet the angle is true; if not 5 feet move one point as much as is necessary to make 5 feet. South American natives estimate height fairly correctly by turning the back to the object, walking straight away from it to the point where the top of the object can be seen by bending over and looking between the legs. Plant a peg at this point and the distance from the peg to the base of the object is roughly equal to the height. Sound travels at the rate of 365 yards every second, as many yards as there are days in the year. By counting the seconds between seeing the flash from a gun, or the steam puff from a locomotive and hearing the sound of the explosion or whistle it is possible to figure the length of the distance between yourself and the gun or locomotive. It is said that the number of seconds between a flash of lightning and the thunder will give the distance between you and the place where the lightning struck. We use weighing machines or scales in buying food, so that we may compare the actual amount of food we buy with a standard weight, otherwise there would be much confusion and business could not be carried on between peoples. For this reason we use pint, quart, peck and bushel measures, all of which are regulated by law as to the amount they hold.