[Illustration: FIG. 26.] The next attempt to measure G, that is the velocity that gravity will produce on a body in a second of time, was made by Attwood, a Cambridge professor. His idea was to weaken the force of gravity and thus make the action slow, not by making it act obliquely, but by allowing it to act, not on the whole, but only on a portion of the mass to be moved. For this purpose he hung two equal weights over a very delicately constructed pulley. Gravity, of course, could not act on these, for any effect it produced on one would be negatived by its effect on the other. The weights would therefore remain at rest. If, however, a small weight _W_, equal say to a hundredth of the combined weight of the weights _A_ and _B_ and _W_, were suddenly put on _A_, then it would descend under an accelerating force equal to a hundredth part of ordinary gravity. We should then have S (the space moved through by the weights) = 1/2 × G/100 × t². With such a system, he found that in 7½ seconds the weights moved through 9 feet. Whence he got 9 = 1/2 G/100 × (7½)². From which G = (2 × 9 × 100)/(7½)² = 32 feet per second nearly. Thus by letting gravity only act on a hundredth part of the total weight moved, namely _A_, _B_, and _W_, he weakened its action 100 times, and thus made the time of falling and the space fallen through sufficiently large to be capable of measurement. To sum up, when a body free to move is acted upon by the force of gravity, its speed will increase in proportion to the time it has been acted upon, and the space it will pass through from rest is proportional to the square of the time during which the accelerating force has acted on it. Gravity is, of course, not the only accelerating force with which we are acquainted. If a spring be suddenly allowed to act on a body and pull it, the body begins to move, and its action is gradually accelerated, just as though it were attracted, and the acceleration of its motion will be proportional to the time during which the accelerating force acts. Similarly, if gunpowder be exploded in a gun-barrel, and the force thus produced be allowed to act on a bullet, the motion of the bullet is accelerated so long as it is in the barrel. When the bullet leaves the barrel it goes on with a uniform pace in a straight line, which, however, by the earth’s attraction is at once deflected into a curve, and altered by the resistance of the air. [Illustration: FIG. 27.]