134 WRITINGS OF JOSEPH HENRY. [1855- 



friction will be produced ; the mechanical collision of the 

 surfaces will set the atoms of the plates in that state of vibra- 

 tion which constitutes heat, and which, if unobstructed, will 

 be communicated to the surrounding setherial medium and 

 radiated to adjacent bodies or off into celestial space. But 

 if detained and applied it may be used to produce changes 

 in matter, such as the boiling of water, the driving of a steam 

 engine, and other objects. Now, if it were possible to collect 

 and concentrate all the impulses of the heat vibrations, and 

 apply them without loss by means of a machine to the eleva- 

 tion of water, the quantity thus raised and the height to 

 which it is raised would be precisely equal to the height and 

 quantity of water, the fall of which produced the first effect. 

 Similarly, if by a steam engine we put in motion the plate 

 of a large electrical machine and disturb the equilibrum of 

 the sether, condensing a portion of it in one part of space and 

 rarefying it in another portion, the force which would be 

 exerted in the restoration of the equilibrium, or in the elec- 

 trical discharge, would be just equal to the amount of energy 

 exerted in producing the coerced condition. If in this case 

 the coerced equilibrium is retained for a day, a year, or a 

 century, so long the amount of energy expended to produce 

 it will, as it were, be locked up but not lost. It will be ready 

 to appear and do work as soon as the detent which prevents 

 the commencement of motion is removed. As a further ex- 

 ample of this, suppose a heavy weight to be elevated b}' steam 

 power to the top of a high pillar, and there placed on an 

 equipoise, so that the least force applied may overturn it 

 and enable it to commence its fall. In its descent it will 

 receive at every instant a new impulse from gravity, and 

 when it arrives at the ground it will expend its accumulated 

 energy in penetrating the surface and in the production of 

 heat, sound, and tremors of the earth. When the weight is 

 resting on the top of the pillar, ready to fall off with the 

 slightest touch, it is said to be in a state of potential energy; 

 and when it has almost reached the earth and is moving 

 with the full velocity of the fall, it has converted its potential 

 energy into actual power. 



