est eventual economic importance. The only sensible 

 thing to do is to give support to all lines of research, 

 however improbable they may seem to be likely to lead 

 to important practical results. 



The first definite knowledge regarding the nature of 

 matter, and of the energy associated with matter, came 

 about the time of Galileo (c. 1590). Galileo investigated 

 the manner in which objects moved when falling, overcom- 

 ing the difficulty of observation of the rapid motion of 

 freely falling bodies by allowing metal spheres to roll down 

 a groove in an inclined plane. In this way the motion was 

 slowed sufficiently for accurate measurement of the position 

 of a moving sphere, at successive instants of time, to be 

 made. Galileo devised the pendulum for measuring short 

 time intervals, having noticed that the period of swing 

 of a pendulum was independent of the amplitude of the 

 swing. By using a short pendulum he was thus able to 

 divide time into a series of equal intervals of length about 

 half a second. These experiments showed that the rate at 

 which a descending sphere increased its speed was exactly 

 the same whatever the weight of the sphere, although 

 a large sphere was of course moved by a larger force (the 

 effective part of its weight down the inclined plane) than 

 was a small one. Since the force of its weight acting on a 

 body to make it accelerate did not affect the magnitude of 

 the acceleration, it was evident that bodies must possess 

 a property enabling them to resist the action of forces 

 acting on them; this property is called inertia. It was 

 simply necessary to assume that the inertia of any body 

 was proportional to its weight to explain the observed 

 constancy of acceleration of all bodies in the same con- 

 ditions. This property of inertia is the fundamental char- 

 acteristic of matter as one constituent of the Universe. 



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