•98 WRITINGS OF JOSEPH HENRY. [1855- 



satisfactory in the explanation, prediction, and control of 

 the phenomena. 



Analogy of heat and sound. — If a heavy cannon be dis- 

 charged at the distance of five or six miles, we shall see the 

 flash almost instantaneously, and in about half a minute 

 after the window will be violently agitated. 



What is the cause of this agitation ? No substance shot 

 from the gun has reached us, for the same effect may be per- 

 ceived on all sides. The simple and true explanation of the 

 phenomenon is that the atoms of air just around the mouth 

 of the piece were for an instant violently pressed outwards 

 by the blast of powder; these atoms were pressed against the 

 next layer, and these against the next, and so on until the 

 impulse reached the distant window. 



Each atom makes a short excursion or vibration, moving 

 but little from its first position, and it is not therefore mat- 

 ter which proceeds from the cannon and produces the dis- 

 tant effect, but a propagation of motion from atom to atom. 



The atoms are endued with inertia, and time is therefore 

 required, even though immense force may be applied, to 

 give them full motion. And again, the atoms are not in 

 contact, but are kept at a distance by repulsion, which in- 

 creases when the atoms are pressed nearer each other. 

 Hence the second layer of atoms does not begin to move 

 with full velocity at the precise moment when motion com- 

 mences in the first. 



The effect would be similar to that which would take place 

 in a series of balls kept apart from each other by helical 

 springs interposed. If a blow were given to the first ball, 

 so as to drive it nearer to the second, the motion would not 

 be instantaneously communicated; the second would resist 

 a change of state, and would not move from its position 

 until the spring was considerably bent. And in this way 

 time would be required to propagate motion from the first 

 ball to the second, from the second to the third, and so on 

 throughout the series. 



If a series of lighter balls were substituted for the first, the 

 springs remaining the same, it is evident the motion would 



