UTILIZATION OF SOLAR ENERGY ACKERMANN. 165 



The boilers were placed at the focus of the reflectors and were 

 covered with a single layer of glass inclosing an air space around the 

 boilers. Each channel-shaped reflector and its boiler was 205 feet 

 long, and there were five such sections placed side by side. The con- 

 centration was 4^ to 1. The maximum quantity of steam produced 

 was 12 pounds per 100 square feet of sunshine, equivalent to 183 

 square feet per brake horsepower, and the maximum thermal efficiency 

 was 40.1 per cent. The best hour's run gave 1,442 pounds of steam at 

 atmospheric pressure, hence, allowing the 22 pounds of steam per 

 brake-horsepower-hour, the maximum output for an hour was 55.5 

 brake horsepower — a result about 10 times as large as anything pre- 

 viously attained, and equal to 63 brake horsepower per acre of land 

 occupied by the plant. A pleasing result was that the output did not 

 fall off muth in the morning and evening. Thus on August 22, 1913, 

 the average power for the five hours' run was no less than 59,4 brake 

 horsepower per acre, while the maximum and mininnim power on that 

 day were 63 and 52,4 brake horsepower per acre, respectively. 



The work of MM. G. Millochau and Ch. Fery was started in 1906 to 

 determine the solar constant and the temperature of the sun. Their 

 v/ork is recorded in Comptes Rendus for 1906 and 1908, and in the 

 Revue Scientifique of September 7, 1907. They give the absolute 

 temperature of the sun as 6,042° C, and the value of the solar constant 

 as 2.38 calories per square-centimeter-minute. This latter value was 

 the result of experiments they made on the summit of Mont Blanc 

 in 1908. 



The article in the Revue Scientifique of September 7, 1907, is by 

 Millochau, and in it he gives the following list of experimenters and 

 the results of their determination of the solar constant, after reading 

 which some may consider the Avord " constant " a misnomer : 



Pouillet, 1837 1. 793 



Forbes, 1842 2.82 



O'Hagen, 1863 1. 9 



Voille, 1875 ._ 2.28 to 2.37 



Langley, 1884 3. 068 



Savelief, 1889 3. 47 



Pertner, 1889 3.05 to 3.28 



Angstroni, 1890 4 



Hansky, 1905 3. 29 



To these we may add : 



Herschel, 1887 1. 98 



Ericsson, 1876 1.93 



Millochau and Fery, 1907 2. 38 



Abbot, 1913 1.93 



In spite of this history of comparative failures, the author is of 

 opinion that the problem of the utilization of solar energy is well 

 worthy of the attention of engineers, for even now it is very nearly 



