PHYSICAL SCIENCE 21 



with his finger. The vessel was filled with water, 

 and the finger removed and replaced at the beginning 

 and the end of the time-interval to be measured. The 

 water flowing out was collected and weighed. 



In this way, Galileo verified by experiment the 

 result of his assumption, and, in this case, no other 

 hypothesis would lead to the same result. Hence 

 the hypothesis that the speed is proportional to the 

 time of fall is confirmed in one instance. Further ex- 

 periments gave similar results, and general belief in 

 the hypothesis as true was established. 



It remained to deal with the actual speed of a 

 body falling freely. This Galileo examined as follows. 

 He showed that a body sliding down one plane would 

 run up another of the same total height whatever 

 be the length of the slope of the planes. Hence it 

 followed that it was the height alone that mattered, 

 and that a body falling freely would acquire the 

 same velocity as a body sliding down an inclined 

 plane, provided that the total height descended were 

 the same in each case. 



But an even more important result followed from 

 the investigation with the two planes. A body 

 sliding down one plane acquired velocity enough 

 to carry it up the other to an equal vertical height. 

 However long were the second plane, and however 

 gently inclined, if friction were negligible, the same re- 

 sult was obtained. It was the vertical rise alone that 

 destroyed the velocity. Hence it followed that, in 



