362 NATURE AND MAN. 



shaft itself an inherent power of motion ; or does it derive that 

 power from any ulterior source ? He sees the shaft apparently 

 terminate in the two end-walls of the building ; and, finding no 

 evidence of its connection with anything else, he may feel himself 

 drawn towards the conclusion that it moves of itself- that is, by 

 the &quot; potency &quot; of its own material constitution. But before 

 adopting this rationale, he sees all the machines stop at once, 

 and finds that the shaft also has ceased to revolve. Here is a 

 new and startling phenomenon. After pondering on it for an 

 hour, and carefully looking out for an explanation, he sees the 

 shaft and its connected machines resume their motion, and yet 

 is certain that no agency visible to him has had any concern in 

 that renewal. By continued watching, he finds this suspension 

 and renewal to be periodical, so that he can frame a law that 

 shall express them in terms of time. Thus he might give a 

 complete phenomenal account of the action of the shaft, which 

 should be perfectly consistent with the assumption of its &quot; inherent 

 potency,&quot; and which might be sufficiently satisfactory to his mind 

 to justify him in believing that there is no more to be learned 

 about it. But not wishing to leave anything uninvestigated, he 

 goes round to the oilier side of the wall. There he finds that one 

 end of the shaft comes through it, and is in mechanical connection 

 with either a steam-engine or a water-wheel ; and by watching 

 what occurs when its motion is checked and renewed, he sees 

 that the Engineer shuts off, or turns on, either the steam generated 

 in the boiler of the steam-engine, or the descending water whose 

 motion drives the wheel. 



I shall not weary the patience of such readers as may have 

 followed me thus far, by tracing out in like detail the further steps 

 of the inquiry ; but shall land them in the final conclusion now 

 accepted by every man of science that the power exerted in both 

 these cases is drawn from solar radiation : the fall of the water 

 which gives motion to the water-wheel, being merely the return 

 of that which has been pumped up as vapour by the sun s heat; 

 whilst the combustion of coal from which steam-power is derived, 

 reproduces, as active force or &quot; energy,&quot; the sunshine that exerted 

 itself during the carboniferous period in dissociating carbonic acid 

 and water into the hydrocarbons of coal and the oxygen of the 



