MECHANICS AND USEFUL ARTS. 79 



convert heat into power, either by steam, hot air, or any other 

 possible means, as it is proved that they iDroduce the full theoret- 

 ical equivalent of cold (negative heat) for the number of foot- 

 pounds emj^loyed; namely, cooling one pound of water one 

 degree for a i^ower equivalent to 700 pounds descending one 

 foot, which, expressed in the adopted scientific manner, is one 

 unit of negative heat for every 700 foot-pounds consumed. 



SOLAR HEAT AS A JliBTIVE POWER. 



M. Mouchot, in a contribution to the " Comptes Rendus," thus 

 speaks of some of his results : — 



" According to my experiments, it is easy to collect, at a cheap 

 rate, more than three-fifths of the solar heat arriving at the sur- 

 face of the globe. The intensity of this calorific source, so feeble 

 in appearance, was revealed by Pouillet, more than 30 years ago. 

 At Paris, a surface of one square metre, normally exposed to the 

 sun's rays, receives, at least, whatever may be the season, during 

 the greater part of a fine day, 10 heat-units (calories) per minute. 

 [The unit of heat adopted by most physicists is the quantity 

 necessary to raise one pound of water from 0° to 1° C. We sup- 

 pose M. Mouchot adopts the same standard.] To appreciate such 

 an amount of heat, it is sufiicient to observe that it will boil, in 

 10 minutes, one litre of water, taken at the temperature of melt- 

 ing ice, and it is almost equal to the theoretical power of a one- 

 horse steam engine. Under the same conditions, a superficies of 

 one *are' (119.603 square yards) would receive, during 10 

 hours of insolation, as much heat as results from the combus- 

 tion of 120 kilogrammes (321,507 pounds troy) of ordinary oil. 

 These numbers are eloquent; they should, if not dispel, at least 

 weaken tlie serious fears entertained by some, in consequence of 

 the rapid exhaustion of coal mines, and the necessity of going to 

 increasingly greater dcjiths, disimting with the subterranean 

 water this precious combustible. The intensity of the calorific 

 radiation of the sun is, moreover, much less at Paris than in inter- 

 trop)ical regions, or upon the elevated plains. It is, therefore, 

 probable, that the invention of ' sun-receivers' will, eome day, 

 enable industry to establish works in the desert, where the sky 

 remains very clear for a long time, just as the hydraulic engines 

 have enabled them to be established by the side of water- 

 courses. 



** Although I have not been able to operate under very favorable 

 circumstances, since my experiments have only been made with 

 the sun of Alencon, Tours, and Paris, I proved, as far back as 

 1861, the possibility of maintaining a hot-air engine in motion, 

 with the help of the sun's rays. INIore lately I have succeeded in 

 boiling, tolerably quickly, several litres of water submitted to in- 

 solation. In short, having satisfied myself that it was sufiicient 

 to have a silver reflector, with a surface of one square metre, to 

 vaporize, in 100 minutes, one litre of water (0.88 quarts), taken 

 at the ordinary temperature, or, in other words, to produce 17 



