158 ANNUAL REPORT SMITHSONIAN INSTITUTION, 193 3 



voir, while the cooler fluid at the bottom of it flows downward to 

 the heat source in replacement. A continuous circulation of hot fluid 

 would thus maintain the temperature of the reservoir. Water, the 

 cheapest liquid, could not be used in the cooker unless under such 

 pressure as would introduce a costly and dangerous element. Tem- 

 peratures approaching 200° C. were desired, and if water were used, 

 steam pressures of nearly 15 atmospheres would prevail. Instead of 

 water, therefore, engine-cylinder oil was preferred. This fluid can 

 be heated to temperatures somewhat above 200° C. without boiling, 

 flashing, or strongly evaporating. 



I chose the cylindrical reflector of ]3arabolic cross-section for the 

 purpose of collecting and focusing the solar rays. A cylindrical 

 reflector does not require, like a conical one, motions about two axes 

 in order to follow the sun. If the cylindrical mirror is mounted 

 upon an axis parallel to the earth's axis, a rotation from east toward 

 west at the same rate as the earth's daily rotation from west toward 

 east is all that is required. To be sure, the full, aperture of the 

 mirror is attained only at the dates of the equinoxes. Yet if the 

 mirror is made rather long from north to south, compared to its 

 width from east to west, this loss is trifling, even in June and 

 December. 



The mirror is made 121/2 feet long and 7^^ feet wide, and is 

 mounted with its long dimension parallel to an axis pointing toward 

 the North Star. For rigidity and cheapness the mirror was framed 

 on five sections of structural steel. 



But how should it become a brightly reflecting mirror ? This was 

 accomplished by using sheets of glossy rolled aluminum, thick enough 

 to preserve their shape when screwed to the steel. They added 

 very little to the weight of the mirror and were found to reflect 

 about 75 percent of the total solar radiation. Experience shows 

 that aluminum sheets retain their highly reflecting surface for 

 several years if kept dry. Later "Allegheny metal ", or stainless 

 steel, was used. 



Having thus made a light, rigid, parabolic cylindrical mirror, the 

 next care was to mount it free to turn about an axis parallel to the 

 earth's. It was necessary to provide a driving mechanism to cause 

 the mirror to follow the apparent daily march of the sun from east 

 to west. Much expense might have been lavished on this part, for 

 the combination of an astronomical driving clock and worm-and- 

 wheel mechanism, usually employed by astronomers to drive tele- 

 scopes of a similar size, often costs hundreds of dollars. A very 

 inexpensive device was used for the mirror of the cooker. A little 

 alarm clock was made master of the situation. It gave a signal 

 which at each 5 minutes allowed the machinery to run. The ma- 

 chinery ran a little too fast, and after a short run was held back 



