collar, terminates above in a hemispherical cap, so that it looks like an enormous thimble, and is covered with a bell-glass of the same shape.
This curious apparatus is nothing else but a solar receiver, or, in other words, a boiler, in which water is made to boil by the heat-rays of the sun. This steam-generator is designed to raise water to the boiling-point and beyond, by means of the solar rays, which are thrown upon the cylinder by the silvered inner surface of the conical reflector. The boiler receives water up to two-thirds of its capacity through a feed-pipe. A glass tube and a steam-gauge communicating with the inside of the generator, and attached to the outside of the reflector, indicate both the level of the water and the pressure of the steam. Finally, there is a safety-valve to let off the steam when the pressure is greater than is desired. Thus the engine offers all desirable safety, and may be provided with all the accessories of a steam-boiler.
The reflector, which is the main portion of the generator, has a diameter of 2.60 metres at its large, and one metre at its small base, and is eighty centimetres in height, giving four square metres of reflecting surface, or of insolation. The interior walls are lined with burnished silver, because that metal is the best reflector of the heat-rays: still brass with a light coating of silver would also serve the purpose. The inclination of the walls of the apparatus to its axis measures 45°. Even the ancients were aware that this is the best form for this kind of metallic mirrors with linear focus, inasmuch as the incident rays parallel to the axis are reflected perpendicularly to the same, and thus give a focus of maximum intensity.
The boiler is of copper, which of all the common metals is the best conductor of heat; it is blackened on the outside, because black possesses the property of absorbing all the heat-rays, just as white reflects them; and it is inclosed in a glass envelope, glass being the most diathermanous of all bodies—that is to say, the most permeable by the rays of luminous heat. Glass further possesses the property of resisting the exit of these same rays after they have been transformed into dark rays on the blackened surface of the boiler. None of these applications of physical laws present any novelty; people reduced them to practice instinctively, as it were, before men of science could assign the reasons. Here the arts of cookery and of gardening, and the processes for warming our rooms, did not wait for the experiments of the physicist. Saussure himself started from these data in his researches; but the inventor needed the discoveries of modern physics in order to give to these applications a rigorous formula.
The boiler proper of the Tours solar engine consists of two concentric bells of copper, the larger one, which alone is visible, having the same height as the mirror, i. e., eighty centimetres, and the smaller or inner one fifty centimetres; their respective diameters are twenty-eight and twenty-two centimetres. The thickness of the metal is only three millimetres. The feed-water lies between the two enve-
