PRIME-MOVERS. 355 



with sulphide of calcium, or other analogous sulphides, makes visible 

 to us the fact that the sun's rays may be stored up as light ; but that 

 they are as truly stored-up (although not in the form of light) in the 

 herb, the tree, and the coal, we also now know, and we appreciate the 

 far-seeing mind of George Stephenson, who astonished his friend by 

 announcing that a passing train was being driven by the sun : and we 

 know that the engineer was right and that the satirical author was 

 wrong, when he instanced as a type of folly the people of Laputa en- 

 gaged in extracting sunbeams from cucumbers. The sunbeams were 

 as surely in the cucumbers as they are in the sulphide of calcium 

 tubes, but in the latter case they can be seen by the bodily eye, while 

 in the former they demand the mind's eye of a Stephenson. 



Although the sailing of ships and the winnowing of grain must from 

 very early times have made it clear that the wind was capable of 

 exercising a substantial power, nevertheless being an invisible agent, 

 it is not one likely to strike the mind as being fit to give effect to a 

 prime-mover, and therefore it is not to be wondered at that prime- 

 movers actuated by water are those of which we first have any record ; 

 unless indeed the toy steam-engine of Hiero may be looked upon as a 

 prime-mover, anterior to those urged by water. It would appear that 

 in the reign of Augustus water-wheels were well known, for Vitruvius, 

 writing at that time, speaks of them as common implements ; not so 

 common, however, as to have replaced the human turnspit, as we 

 gather from his writings that the employment of men within a tread- 

 wheel was still the more ordinary mode of obtaining a rotary force. 

 It would seem, however, that water-wheels driven by the impact of the 

 stream upon pallet boards were employed in the time^ of Augustus not 

 merely to raise water by buckets placed about the circumference of the 

 wheels, but also to drive millstones for grinding wheat. Strabo states 

 that a mill of this kind was in use at the palace of the King of Pontus. 



Having thus mentioned the earliest record of hydraulic (or indeed 

 of any) prime-movers, I will not endeavour to trace their history down 

 to modern times, as it would be impossible to do so usefully within 

 the limits of an address. I will therefore, without further reference to 

 the historical part of the subject, ask you to join me in considering 

 \vhat are the conditions which govern the application of water to 

 hydraulic prime-movers. 



