INDUSTRIAL APPLICATIONS OF SOLAR HEAT. 557 



at Tours, does not exceed fifteen hundred francs, and, when the manu- 

 facture is carried on upon a large scale, will he much less. By sub- 

 stituting for the silver plate, which is the most costly portion of the 

 reflector, brass with a thin coat of silver, which will serve the pur- 

 pose equally well, a considerable reduction of cost is effected. 



As the insolation surface, and consequently the power of the ap- 

 paratus, is quadrupled when the diameter of the mirror is doubled, it 

 will be easy to construct large generators without adding very much 

 to the cost or complicating the mechanism. The one thing to be 

 avoided in this case will be too great intensity of heat. It cannot be 

 objected that the conical reflector takes up too much room, for a com- 

 mon steam-engine occupies considerable space likewise with its long 

 boilers and its high chimney ; as for the motor, properly so called, 

 and the contrivances for transmitting the power, they are the same in 

 both cases. 



The strongest winds, at least in our latitudes, have no action on 

 the reflection of the solar heat, or upon the mirror itself, which is not 

 shaken by them. This is an important point, for this is an apparatus 

 which must always be exposed in the open air. In regions where the 

 wind-storms are more severe than they are here, the reflector might 

 be staid and strengthened with iron ribs, so as to resist the most 

 violent cyclones. It has been demonstrated that the bell-glass, even 

 when highly heated by the direct radiation from the boiler, is in no 

 danger of breaking, even when a cold rain falls upon it, and that it 

 is even proof against hailstones; and now that a process has been . 

 invented for tempering glass and making it almost unbreakable, we 

 can without difficulty obtain bell-glasses strong enough for any emer- 

 gency. 



Experience will hereafter lead to many improvements now un- 

 thought of; but even as it stands to-day the solar engine at Tours is 

 ready to pass from the speculations of theory to the application of 

 practice. It is neither over-costly, nor difficult to set up, nor so com- 

 plicated as to require great skill in managing it ; and, from whatever 

 point of view we regard it, it meets and overcomes all objections. 

 We may say that it lends itself to every industrial use in which solar 

 heat can be employed, especially in ' tropical countries where the ab- 

 sence of all kinds of fuel for industrial uses is severely felt. In the 

 not distant future, in other countries, too, there will exist no other fuel 

 than the sun, no other engines than those driven by solar heat. By that 

 time no doubt the means of storing up this heat will have been dis- 

 covered, for in our latitudes we shall have to make provision against 

 cloudy days and seasons of rain, which unfortunately constitute the 

 major part of the year. 



It may appear to be a pleasant paradox to say that future genera- 

 tions, after the coal-mines have been exhausted, will have recourse to the 

 sun for the heat and energy needed in manufacture and in domestic 



