52 ELEMENTS OF LABORATORY WORK 



CHAPTER IV 



OBSERVATIONS OF CERTAIN MUTUAL CHANGES COMMON 

 TO ALL KINDS OP MATTER 



36, Bodies displaced Equally from the Surface of the Earth 

 reach it again Simultaneously if allowed to Fall. A large 

 electro-magnet may be used for suspending two bodies" at the 

 same height. When the current is disconnected the bodies fall 

 at the same rate. One body may be of iron, the other of wood 

 with a piece of iron fixed so that the magnet will hold it. A 

 piece of paper between the ends of the electro- magnet and the 

 bodies ensures their simultaneous detachment. Variation, either 

 in the quantity or kind of matter, does not change the result. 

 All bodies let fall at the same instant and from the same 

 height reach the earth at the same time, except so far as the 

 obstruction of the air may intervene. Bodies which vary 

 in falling in the air are found to be alike when tried in 

 vacuo. If the displacement is varied the statement will be 

 found to hold good. 



In this observation we have assumed the earth to remain 

 at rest, while the two small bodies are said to be moving. By 

 making this assumption, which our definition of movement 

 justifies, we simplify all such investigations. The system under 

 observation contains the earth and these two bodies. No 

 change in the surroundings, however varied or repeated, has 

 ever been found to annul either the power of return of any 

 given body when removed from the earth, or the equal rate 

 of return of two bodies equally removed. It is clear, then, 

 that the earth and such bodies are alone concerned in this 

 special change, and they constitute a material system in which 



