UTILIZATION OF SOLAR ENERGY — ACKERMANN. 155 



1878). He started on the work in 1876, and his experiments led him 

 to conchide, as did Bufl'on, that silvered-glass mirrors were superior 

 to polished-metal ones. This is no doubt true for ordinary use, 

 though for laboratory experiments the polished-metal ones give better 

 results, as there is then no absorjation by the glass (pi. 2). 



In two particulars Adams was much at fault — (1) in believing 

 that the solar rays which reach the earth are not practically parallel, 

 and this in spite of the opposite opinions of the many physicists 

 whom he quotes, and (2) in believing that the temperature attained 

 at the focus of a lens or mirror is directly proportional to the con- 

 centration of the rays. As a consequence, he stated that if a lens 85 

 feet 4 inches in diameter concentrated the radiation onto a circle one- 

 lialf inch in diameter the temperature would be 73,400,320° F. This is 

 equal to 10,780,000° C, while the temperature of the sun itself is only 

 6,000° C, and no amount of such concentration could produce a 

 temperature in excess of this. This error on the part of Adams and 

 Pope seems to be due to a confusion of " temperature " with " quan- 

 tity of heat." 



His experiments were all made with plane or flat glass mirrors, 

 the use of which he strongly advocated in preference to curved metal 

 ones, which Mouchot used. Sometimes he used groups of 18 mirrors, 

 each 17 by 10-1 inches, and sometimes of 32, each 9 by 6 inches. The 

 latter he arranged in a concave wooden frame in 4 tiers of 8 in each 

 tier. Such a group of 32 formed 1 unit, of which he had 16, all 

 focused onto one boiler. When placed together the 16 units formed 

 a portion of the surface of a hollow sphere 40 feet in diameter. One 

 of his boilers was of copper one-sixteenth inch thick, 16 inches diam- 

 eter, 2 feet 7 inches high, and held 9 gallons of water, which boiled 

 in 30 minutes and evaporated 3f gallons in an hour. 



His next boiler was also of copper one-fourth inch thick, and of 

 the same design and external dimensions as Mouchot's, but with a 

 water space between the inner and outer shells of 3 inches instead 

 of 3 centimeters, and containing 12 gallons of water as compared 

 with Mouchot's 4| gallons. The 12 gallons of water were boiled and 

 the pressure raised to 10 pounds to the square inch in the half hour 

 from 7.30 a. m. to 8 a. m., and by 8.30 a. m. the pressure was 70 pounds 

 to the square inch, when the safety valve opened, whereupon he goes 

 on to say: 



A gentleman present kept the valve clown by placing his foot on it, till the 

 steam, escaping from several leaks in the joints of the fittings made the position 

 imtenable. The weiglit on the safety valve was then supplemented by a brick 

 suspended from the lever by a piece of string, when suddenly the packing and 

 red lead at the top of the dome under the socket of the steam pipe (which had 

 been fixed by my butler, who professed to have formerly been a fitter) gave 

 way, and, with a terrific noise, the whole volume of steam rushed out of the 



