A rising body decreases thirty-two feet in velocity for every second of time it is in rising. If it be three seconds in rising, its velocity will be eighty feet the first second, forty-eight feet the second second, and sixteen feet the third second, being just the reverse of the same body when falling, as is shown in Fig. 29. [Illustration: Fig. 29.] How may the distance a body falls in a given time be found? It may be found by multiplying the number of seconds it is in falling, by the same number, and this product by sixteen feet. Thus, when it is three seconds in falling, multiply three by three, and this product by sixteen feet, which gives one hundred and forty-four feet, the distance it falls in three seconds. What familiar example have we of the force of falling bodies? We have an example in the large hail-stones which fall to the earth in summer-time, breaking the glass in our windows and doing much injury to the crops. Centre of Gravity. What is the centre of gravity in a body? The _centre of gravity_ in a body, is the point on which the body may be balanced, as is shown in Fig. 30. [Illustration: Fig. 30.] Where is the centre of gravity in a circle? It is the centre of the circle. Where is the centre of gravity in a ball? It is the centre of the ball. What influence does the earth have on the centre of gravity? The earth draws the centre of gravity in a body as near to itself as possible. Why will an egg, when placed on either end, fall over on its side? Because the centre of gravity is nearer the earth when the egg is placed upon its side than when upon either end, as is shown in Fig. 31 and Fig. 32. [Illustration: Fig. 31.] [Illustration: Fig. 32.] Why will a ball remain in any position when placed on the earth? Because its centre of gravity is always the same distance from the earth, whatever may be its position. When will a body change its position if left unsupported? A body will change its position whenever by so doing its centre of gravity will be lowered. When may a body be readily moved about? When its centre of gravity always remains at the same distance from the earth while being moved. Why is a round body, like a ball, easily rolled about the floor? Because its centre of gravity remains at the same distance from the floor. When is the position of a body difficult to change? Whenever the centre of gravity in it has to be lifted up in order to make the change, as is shown in Fig. 35. [Illustration: Fig. 33.] [Illustration: Fig. 34.] [Illustration: Fig. 35.] The centre of gravity, in Fig. 33, may be lowered; hence, it will change its position if unsupported. The centre of gravity, in Fig. 34, can neither be lowered nor lifted up; hence, it may be readily rolled about. The centre of gravity, in Fig. 35, must be lifted up; hence, it remains firm in its place. When does an object stand firm?