PRIME-MOVERS. 363 



frequently obscured, and secondly, from the fact that a large portion 

 of the energy of the sun is spent in evaporating moisture from the 

 ground and not in the direct production of combustible material. I 

 have found it extremely difficult to obtain any trustworthy data as to 

 the weight of fuel grown per acre per annum. If we take the sugar 

 cane we find that in extremely favourable cases as much megass and 

 sugar are produced per acre as together would equal in calorific effect 

 about five tons of good Welsh coal. Coming to our own country and 

 dealing with a field of wheat, the wheat and straw together may be 

 taken as being equal probably to about two tons of coal as a 

 maximum. The statements made to me with regard to the production 

 of timber per acre per annum, when grown for the purpose of burning, 

 are very various, but the best average I can make from them is that in 

 this country there is produced as much wood as is equal in calorific 

 effect to about one and a half tons of good coal per acre. Com- 

 paring these productions of heat-giving material with the energy of the 

 sun, as shown in the evaporation of water, one sees how tempting a 

 field is that of the direct employment of the solar rays as a source 

 of power, more especially when it is remembered that those rays are 

 obtained from week to week and year to year without having to wait 

 the tardy growth of the fuel-destined tree. 



I will now ask you to consider with me the prime-movers that owe 

 their energy to the heat developed by the combustion of some ordinary 

 kind of fuel, coal or wood. Passing by as a mere toy and as being 

 not an actual prime-mover, the reactionary steam sphere, the seolipile 

 of Hiero, I will come at once to those simple forms of heat engine, 

 intended, whether worked by steam or by the expansion of air, for the 

 raising of water. Salomon de Caus, in his work of 1615, already 

 mentioned, says that if a globe be filled with water and have in its 

 upper part a pipe dipping nearly to the bottom, and if the globe be 

 put upon the fire the heat will cause the expansion of the contents, 

 and the water will be delivered in a jet out of the tube. 



The Marquis of Worcester in his " Century of Inventions," pub- 

 lished in 1659, makes, as is well known, a similar proposition, but it 

 does not appear that these machines were seriously contemplated for 

 practical use. Papin (I take Belidor's article, No. 1276, as my 

 authority), in 1698 (as appears in his pamphlet of 1707), experimented 



