=94. Falling Bodies.=--One of the earliest physical facts learned by a child is that a body unsupported falls toward the earth. When a child lets go of a toy, he soon learns to look for it on the floor. It is also of common observation that light objects, as feathers and paper, fall much slower than a stone. The information, therefore, that all bodies actually fall at the same rate in a vacuum or when removed from the retarding influence of the air is received with surprise. This fact may be shown by using what is called a coin and feather tube. On exhausting the air from this tube, the feather and coin within are seen to fall at the same rate. (See Fig. 78.) when air is again admitted, the feather flutters along behind. [Illustration: FIG. 78.--Bodies fall alike in a vacuum.] =95. Galileo's Experiment.=--The fact that bodies of different weight tend to fall at the same rate was first experimentally shown by Galileo by dropping a 1-lb. and a 100-lb. ball from the top of the leaning tower of Pisa in Italy (represented in Fig. 79). Both starting at the same time struck the ground together. Galileo inferred from this that feathers and other light objects would fall at the same rate as iron or lead were it not for the resistance of the air. After the invention of the air pump this supposition was verified as just explained. [Illustration: FIG. 79.--Leaning tower of Pisa.] =96. Acceleration Due to Gravity.=--If a body falls freely, that is without meeting a resistance or a retarding influence, its motion will continually increase. The _increase_ in motion is found to be constant or uniform during each second. This uniform increase in motion or in velocity of a falling body gives one of the best illustrations that we have of uniformly accelerated motion. (Art. 75.) On the other hand, a body thrown upward has uniformly retarded motion, that is, its acceleration is downward. The velocity acquired by a falling body in unit time is called its _acceleration_, or the _acceleration due to gravity_, and is equal to 32.16 ft. (980 cm.) per second, downward, each second of time. In one second, therefore, a falling body gains a velocity of 32.16 ft. (980 cm.) per second, downward. In two seconds it gains twice this, and so on. In formulas, the acceleration of gravity is represented by "_g_" and the number of seconds by _t_, therefore the formula for finding the velocity, _V_,[F] of a falling body starting from rest is _V_ = _gt_. In studying gravity (Art. 89) we learned that its force varies as one moves toward or away from the equator. (How?) In latitude 38° the acceleration of gravity is 980 cm. per second each second of time. [F] _V_ represents the velocity of a falling body at the end of _t_ seconds.