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coming of it. When the change is once begun, the requisite help is 

 given to the succeeding parts by those which have gone off first. 



Again, water confined in a high reservoir is not free to go to a 

 lower one ; although a siphon, primarily filled with water, may help 

 the parts successively over the obstacle by lending to each the requi- 

 site mechanical work in advance, which it afterwards pays to the parts 

 which are to follow, besides that it gives out in its fall a great addi- 

 tional amount of power or energy applicable otherwise. Two reser- 

 voirs of water, on the same level, and having an opening between 

 them under the water surface, would represent the case of perfect 

 freedom for change of state ; and two on a level with one another, 

 but separated by a partition, would represent the case in which no 

 mechanical work would finally be either given out or absorbed by the 

 change, but in which there is not perfect freedom to change, until a 

 siphon or other means of help is applied. 



A bell hung from an axle and then turned up, and left resting 

 against a stop a little beyond its position of unstable equilibrium, is 

 not free to go down, but a slight pull will bring it over this position 

 and make it free to swing, which the work stored as potential energy 

 in the raising of it from its low or hanging position, will cause it to 

 do ; its fall till it comes to the bottom being essentially accompanied 

 by the loss of that potential energy, as such, though not as actual 

 energy, out of the system of which it and the earth are the two parts, 

 and in which change of their distance asunder constitutes change of 

 their potential energy. 



If in an atmosphere of steam resting on water at its boiling tem- 

 perature for the pressure of the steam ; as, for instance, in the inside 

 of a boiler partly filled with water, and partly with steam, an inverted 

 cup, or bell-shaped vessel, be suspended, and if it then, being full of 

 steam, be forced down under the water, mechanical work will be im- 

 parted as potential energy to the system of which the steam and 

 water in the boiler form one part, and the earth is the other part ; 

 though, for brevity of expression, the work may be spoken of as 

 applied to the steam and water. In this case there is perfect freedom 

 for the steam forced under the water to condense and cause by com- 

 munication of its latent heat the generation of an equal quantity of 

 steam at the surface of the water under which the bell was sunk. 



