150 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1915. 



than desired. Tlius, 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 meters at its large and 1 meter at its small base, and is SO centimeters in 

 height, giving 4 square meters of reflecting surface or of insolation. The in- 

 terior 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 par- 

 allel to the axis are reflected perpeudiculai'ly 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 mast 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 trans- 

 formed into dark rays on the blackened surface of the boiler. 



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., 80 centimeters, and the smaller or inner one 50 centimeters. Their 

 respective diameters are 28 and 22 centimeters. The thickness of the metal is 

 only 3 millimeters. The feed water lies between the two envelopes, forming an 

 annular envelope 3 centimeters in thickness. Thus the volume of liquid is 20 

 liters, and the steam chamber has a capacity of 10 liters. The inner envelope is 

 empty. Into it pass the steam pipe and the feed pipe of the boiler. To the 

 steam pipe are attached the gauge and the safety valve. The bell glass covering 

 the boiler is 85 centimeters high, 40 centimeters in diameter, and 5 millimeters 

 in thickness. There is everywhere a space of 5 centimeters between its walls 

 and those of the boiler, and this space is filled with a laj^er of very hot air. 



Mechanism was provided whereby the reflector was adjusted by 

 hand to follow the movement of the sun. 



On May 8, 1875, a fine day, 20 liters of water, at 20° C, introduced into the 

 boiler at 8.30 a. m., produced steam in 40 minutes at 2 atmospheres (30 pounds) 

 of pressure to the square inch, i. e., a temperature of 121°, or 21° above boiling 

 water. The steam was then raised rapidly to a pressure of 5 atmospheres (75 

 pounds to the square inch), and if this limit was not exceeded it was because 

 the sides of the boiler were only 3 millimeters thick, and the total effort sup- 

 ported by these sides was then 40,000 kilograms. It would have been dangerous 

 to have proceeded further, as the whole apparatus might have been blown to 

 pieces. 



Toward the middle of the same day, with 15 liters of water in the boiler, the 

 steam at 100° — that is to say, at a pressure of 1 atmosphere — rose in less than 

 a quarter of an hour to a pressure of 5 atmospheres, equal to a temperature of 

 153°. Finally, on July 22, toward 1 p. m., an exceptionally hot day, the appa- 

 ratus vaporized 5 liters of water per hour, which is equal to a consumption of 

 140 liters of steam per minute, and one-half horsepower. For these experiments 

 the inventor used an engine which made 80 strokes per minute under a con- 

 tinued pressure of 1 atmosphere. Later on it was changed for a rotative 

 engine — that is to say, an engine with a revolving cylinder — w^hich worked 

 admirably, putting in motion a pump to raise water, until the pump, which 

 was too weak, was broken. 



