at Tours, does not exceed fifteen hundred francs, and, when the manufacture is carried on upon a large scale, will he much less. By substituting for the silver plate, which is the most costly portion of the reflector, brass with a thin coat of silver, which will serve the purpose equally well, a considerable reduction of cost is effected.
As the insolation surface, and consequently the power of the apparatus, 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 common 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 emergency.
Experience will hereafter lead to many improvements now unthought 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 complicated 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 absence 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 discovered, 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 generations, after the coal-mines have been exhausted, will have recourse to the sun for the heat and energy needed in manufacture and in domestic
