404 CORRELATION OF PHYSICAL AND VITAL FORCES. 



only the same mechanical force acting through a different 

 kind of instrumentality ; the strokes of the piston of the 

 steam-engine being dependent upon the elastic force of the 

 vapour of water, whilst the revolution of the water-wheel is 

 maintained by the downward impetus of water en masse. 

 But to what antecedent dynamical agency can we trace these 

 forces ? That agency in each case is Heat ; a force altogether 

 dissimilar in its ordinary manifestations to the force which 

 produces sensible motion, yet capable of being in turn con- 

 verted into it and generated by it. For it is from the Heat 

 applied beneath the boiler of the steam-engine that the non- 

 elastic liquid contained in it derives all that potency as elastic 

 vapour, which enables it to overcome the vast mechanical re- 

 sistance that is set in opposition to it. And, in like manner, 

 it is the heat of the solar rays which pumps up terrestrial 

 waters in the shape of vapour, and thus supplies to Man a 

 perrenial source of new power in their descent by the force 

 of gravity to the level from which they have been raised.* 



The power of the steam-engine, indeed, is itself derived 

 more remotely from those same rays ; for the Heat applied 

 to its boilers is but the expression of the chemical change in- 

 volved in combustion ; that combustion is sustained either by 

 the wood which is the product of the vegetative activity of 

 the present day, or by the coal which represents the vegeta- 

 tive life of a remote geological epoch ; and that vegetative 

 activity, whether present or past, represents an equivalent 

 amount of Solar Light and Heat, used up in the decomposi- 

 tion of the carbonic acid of the atmosphere, by the instru- 

 mentality of the growing plant.f Thus in either case we 

 come, directly or indirectly, to Solar Radiation as the main- 



* See on this subject the recent admirable address of Sir William Arm- 

 strong at the Meeting of the British Association at Newcastle. 



f This was discerned by the genius of George Stephenson, before the 

 general doctrine of the Correlation of Forces had been given to the world 

 by Mayer and Grove. 



