UTILIZING THE SUn's ENERGY. 267 



"Due consideration can not fail to convince us that the rapid ex- 

 haustion of the European coal tiekls will soon cause great changes 

 with reference to international relations in favor of those countries 

 which are in possession of continuous sun power. Upper Egypt, for 

 instance, will, in the course of a few centuries, derive signal advantage 

 and attain a high political position on account of her perpetual sun- 

 shine and the consequent command of unlimited motive force. The 

 time will come when Europe must stop her mills for want of coal. 

 Upper Egypt, then, with her never-ceasing sun power, will invite the 

 European manufacturer to remove his machinery and erect his mills 

 on the firm ground along the sides of the alluvial plain of the Nile, 

 where an amount of motive power may be obtained many times greater 

 than that now employed by all the manufactories of Europe." 



The probable value of the quantity of energy transmitted to the 

 earth from the sun, according to the conclusion, after extended inves- 

 tigation of the late Prof. De Volson Wood, the greatest of American 

 thermodynamists of the nineteenth century, is not far from that 

 obtained b}^ Langley — 133 foot-pounds per square foot of receiving 

 area per second, about 133 550 = 0.2-1 horsepower, or the equivalent of 

 4 square feet per horsepower.* As actually utilized, Ericsson 

 reported his solar engine to supply a horsepower from 100 square 

 feet of receiving area, on a bright, clear da}', and other experimen- 

 talists, with apparenth' less efficient apparatus, report a horsepower 

 from about 150 square feet in sunshine. 



This figure is confirmed by recent experiments at Passadena, Cal., 

 where it is said that the etficienc}' reached b}^ Ericsson has in some 

 cases been attained. The California apparatus includes a truncated 

 conical mirror. 33 feet 6 inches in diameter at the top and 15 feet at 

 the bottom, which concentrates the ra^'s of the sun received upon its 

 1,788 facets at a focus where a boiler is placed, and where steam is 

 made, to operate a steam engine of small power. The whole mass of 

 glass and iron composing the mirror is moved by a suitably arranged 

 clock, and is automatically held with its axis directed toward the sun. 

 The boiler is carried on the same frame and moves with the mirror. 

 It is 13 feet G inches In length, and contains aboi^t 10 cubic feet of 

 water and 8 cubic feet of steam space. The steam pressure is carried 

 at 150 pounds per square inch. It is rated at 10 horsepower. This 

 power is utilized in pumping water, but the reported figures are 

 inconsistent with its rating. To set the machine in operation it is 

 only necessary to turn the apparatus by hand until its axis points at 

 the sun's disk and to set the clockwork in operation. To stop it 

 requires simply the turning of the mirror away from the sun and the 

 stopping of the machinerj' which adjusts it. 



* Wood employs this value in his classic and remarkable paper "On the Luminif- 

 erous Ether," the first rational determination of the physical properties of the ether, 

 and a most important and imjiressive work. Phil. Trans. Magazine, November, 

 1885. App. to Wood's Thermodynamics; N. Y., 1887. J. Wiley & Sons. 



