554 THE POPULAR SCIENCE MONTHLY. 



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 re- 

 flecting surface, or of insolation. The interior walls are lined with 

 burnished silver, because that metal is the best reflector of the heat- 

 ravs : still brass with a lisrht 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 pos- 

 sesses 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 w T arming 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 con- 

 centric 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 iifty 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- 



