-1859] WRITINGS OP JOSEPH HENRY. 131 



agitation of the setherial medium ; and these together are 

 equal to the energy generated by the reciprocal motion of 

 the atoms. If by any means a force were applied to separate 

 the atoms to the same distance at which they were at first, 

 this force would be just equal to that due to the rushing 

 together of the atoms. Two atoms separated, and in a con- 

 dition to be violently drawn together, are said to be in a 

 state of energy or power ; but when they have entered into 

 combination, they are then in a state of inertness. The 

 same may be said of a weight elevated above the surface of 

 the earth. A certain amount of muscular power must be 

 exerted to overcome the attraction of gravitation, and to raise 

 the weight to the given height, say ten feet. It is then in a 

 state of power, or in a condition to produce permanent 

 changes in matter, and other effects which we technically 

 denominate "work." 



The energy developed in the weight may be employed to 

 drive a pile into the ground, or it may be made to turn a mill 

 and grind corn; but the work done in these two cases, when 

 properly measured, will be the same, and just equal to that 

 expended in elevating the weight. If the weight be raised 

 to double the height, twice the force will be expended in ac- 

 complishing this effect, and the weight in its descent to the 

 earth will also do a corresponding amount of work. The 

 explanation of the development of the energy exhibited in the 

 fall of a body from a height will be plain when we consider 

 that gravity acts on the mass with a force proportioned to 

 the number of pounds in weight at every point in its des- 

 cent; and if we suppose that in the first this attraction gave 

 it a certain velocity, and gravity were then to cease, the body, 

 on account of its inertia, would continue to descend with 

 this velocity to the end of its course. But if the attraction 

 continues to act, new impulses are imparted at every instant, 

 and the velocity will continually increase until it reaches the 

 ground, where it will produce an effect which is the equiva- 

 lent of the power accumulated in its descent. The mechan- 

 ical energy of matter therefore is measured by the distance 

 of the atoms into the intensity of the attraction at the differ- 



