54 



WELLS'S NATURAL PHILOSOPHY. 



If ten or a hundred leaden balls be disengaged together, they -will fiHl in 

 the same time, and if they be molded into one ball of great magnitude, it 

 will still fall in the same manner. 



102. Hence all bodies under the influence of gravity 

 alone, must fall with equal velocities.'"' 



„ , . There are some familiar facts which seem FiQ. 27. 



By what ex- , . , 



periment can to be Opposed to this law. "When we let go ^^ 



oil prove this 



law? 



a feather and a mass of lead, the one floats 

 in the air, and the other falls to the ground very 

 rapidly. But in this case, the operation of gravity is modified 

 by the resistance of the air ; the feather floats because the 

 air opposes its descent, and it can not overcome the resistance 

 offered. But if we place a mass of lead and a feather in a 

 vessel exhausted of air, and Uberate them at the same time, 

 they will fall in equal periods. The experiment is easily 

 shown by taking a glass tube. Fig. 27, closed at one end, and 

 Bupphed with an air-tight cap and screw-cock at the other. 

 A feather and a piece of metal are previously inclosed in the 

 tube. The tube being filled with air, and inverted, the metal 

 will fall with greater speed than the feather, as might be ex- 

 pected. If the tube be now exhausted of air by means of 

 an air-pump and the screw-cock, and in this condition in- 

 verted, the feather and the metal will faU from end to end 

 of the tube with equal velocity. 



103. If a man leap from a chair or table, 

 he will strike the ground without injury. If 

 the same man leap from the top of a high 

 house, he will probably be killed. These, 

 and many like instances, prove that the force 

 with which a falling body strikes the ground depends upon 

 the height from which it falls. But the force depends on 

 the velocity of the body the moment it touches the ground ; 

 therefore, the velocity \nth which a body falls depends also 

 upon the height from which it descends. 



Upon what do 

 the force and 

 velocities of 

 falling bodies 

 depend 1 



• Previous to the time of Galileo, the philosophers maintained that the velocity of i 

 falling body was in proportion to its WL'ight, and that if two bodies of unequal weights, 

 were let fall from an elevation, at the same moment, the heavier would reach the ground 

 as much sooner than the lighter, as its weight exceeded it. In other words, a body weigh- 

 ing two pounds would fall in half the time that would be required by a body weighing ono 

 pound. G.ilileo, on tlie contrary, asserted that the velocity of a falling body is iudependaut 

 of its weight, and not affected by it. The dispute running high, and the opinion of the 

 public being generally averse to the views of Galileo, he challenged his opponents to test 

 the matter by a public experiment. The challenge was accepted, and the celebrated leaning- 

 tower of Pisa agreed upon as the placi of trial. In the presence of a large concourse, two 

 balls were selected, one having exactly twice the weight of the other. The two were then 

 dropped from the summit of the tower at the same moment, and, in exact accordance 

 with the asaertions of Galileo, they both struck the ground at the same instant. 



