UTILIZATION OF SOLAR ENERGY — ACKERMANN. 153 



minute for each square foot of area presented perpendicularly to the 

 sun's rays." Three and five-tenths B. t. u. per square-foot-minute 

 =0.95 calories per square-centimeter-minute. The mean transmis- 

 sion of solar radiation by the atmosphere over a zenith distance from 

 45° E. to 45° W. is 67.5 per cent when the sky is clear. Thus 

 0.675X1-93= 1.31 calories per square-centimeterrminute are available 

 at the earth's surface. Hence the eftlciency of Ericsson's boiler was 

 ^tI^X 100=72.5 per cent, which is remarkably high. 



In 1872 Ericsson built his hot-air solar engine, which had a reflector 

 the shape of which was approximately a portion of a sphere and 

 which concentrated the solar radiation onto one end of the cylinder. 

 The power of both these engines was evidently very small. On July 

 9, 1875, Ericsson wrote that he had up to that time constructed and 

 started seven sun motors. 



Ericsson wrote in Nature of January 3, 1881, an illustrated article 

 describing another of his sun motors which he erected in New York 

 in 1883, in spite of his opinion as to the cost of solar steam (previ- 

 ously quoted) expressed in 1878 (pi. 1). His description was as fol- 

 lows: 



The leading feature of the smi motor is that of concentrating the radiant 

 heat bj^ means of a rectangular trougli liaving a curved bottom lined on the 

 inside with polished plates so arranged that they reflect the sun's rays toward 

 a cylindrical heater placed longitudinally above the trough. This heater, it is 

 scarcely necessary to state, contains the acting medium, steam or air, employed 

 to transfer the solar energy to the motor, the transfer being effected by means 

 of cylinders provided with pistons and valves resembling those of motive engines 

 of the ordinary type. Practical engineers, as well as scientists, have deniou- 

 stratod that solar energy can not be rendered available for producing motive 

 power, in consequence of the feebleness of solar radiation. The great cost of 

 large reflectors and the difficulty of producing accurate curvature on a large 

 scale, besides the great amount of labor called for in preventing the polished 

 surface from becoming tarnished, are ob.1ections which have been supposed to 

 render direct solar energy practically useless for producing mechanical power. 



The device under consideration overcomes the stated objections by very 

 simple means, as will be seen by the following description : The bottom of the 

 rectangular trough consists of straight wooden staves, supported by iron ribs of 

 parabolic curvature secured to the sides of the trough. On these staves the 

 reflecting plates, consisting of flat window glass silvered on the under side, are 

 fastened. It will be readily understood that the method thus adopted for 

 concentrating the radiant heat does not call for a structure of great accuracy, 

 provided the wooden staves are secured to the iron ribs in such a position that 

 the silvered plates attached to the same reflect the solar rays toward the heater. 



Referring to the illustration, it will be seen that the trough, 11 feet long and 

 16 feet broad, including a parallel opening in the bottom, 12 inches wide, is 

 sustained by a light truss attached to each end, the heater being supported by 

 vertical plates secured to the truss. The heater is Qi inches in diameter, 11 

 feet long, exposing 130X9.8=1.274 superficial inclios to the action of the reflected 

 solar rays. The reflecting plates, each 3 inches wide and 26 inches long, inter- 

 cept a sunbeam of 130X180=23,400 square inches section. The trough is sup- 

 ported by a central pivot, round which it revolves. The change of inclination 



