-1859] WRITINGS OF JOSEPH HENRY. 99 



be transmitted sooner, because the inertia would be in pro- 

 portion to the weight of the balls. Hence sound is trans- 

 mitted more rapidly in lighter than in heavier gases; in 

 h^'drogen its velocity is greater than in carbonic acid. 



Again, we may suppose the stiffness of the springs to vary, 

 or in other words, the repulsion between the atoms to become 

 greater or smaller. If the springs become stiffer, then it is 

 evident the motion will be transmitted sooner, for if the 

 springs were infinitely rigid, or what is the same if a per- 

 fectly solid body were interposed between the balls, then the 

 first ball could not move without at the same moment giving 

 motion to the last. Hence if we increase the elasticity of a 

 medium and at the same time diminish the size of its atoms 

 any required velocity can be attained. Now though the 

 flash is apparently perceived at the same instant at different 

 places on the surface of the earth, yet we know from the 

 most satisfactory evidence that this is really not the case, 

 and that light and heat, as well as sound, require time for 

 their propagation. Every impulse at the sun requires about 

 eight minutes before it is felt at the distance of the earth. 



The analogy between light and sound does not cease here; 

 and to exhibit the resemblance still further, let us suppose a 

 large bell placed in mid-air to be struck a single blow 

 with a heavy hammer; we know that the lower rim of metal 

 will be thrown into a state of vibration ; it will be com- 

 pressed into an elliptical form, the shorter axis in the 

 direction of the blow. The elasticity will bring it back 

 to its normal state, and will then carr}^ it beyond in the 

 other direction; and thus the part of the bell which 

 is struck will continue to move backward and forward 

 rapidly for a considerable time, which would be indefinitely 

 prolonged were the experiment made in a perfect vacuum, 

 and were no change produced in the atoms of the metal. 

 In open air however the motion becomes feebler and feebler, 

 and after a few minutes dies away and entirely ceases. The 

 principal cause of this diminution is evidently the impart- 

 ing of the motion of the metal to the immediately surround- 

 ing atoms of the air, and these to the next, and so on. It 



