152 ■ ANNUAL EEPOET SMITHSONIAN INSTITUTION, 1915. 



many times more costly than steam produced by burning coal. (Letter dated 

 Sept. 21, 1878, to R. B. Forbes.) 



We have already referred to his remarkably accurate determination 

 of the solar constant ; but he was not so happy in deducing the tem- 

 perature of the sun, which he made to be 723,000° C, the present 

 accepted result being only 6,000° C. 



He tried hot-air engines, as well as steam engines, for utilizing solar 

 energ)^, and claimed that the steam engine which he constructed in 

 New York for this purpose in 1870 was the first one driven by the 

 direct agency of solar radiation. The diameter of its cylinder was 

 4^ inches. He afterwards modified his solar hot-air engine so that 

 it might be used as a small pumping engine, using gas as its heat 

 supply. 



The profits upon this chip from his workshop are already estimated at several 

 times the amount of the f20,(X)0 expended by Ericsson upon the solar investiga- 

 tions leading up to this invention. (Vol. II, p. 275 of his " Life," by W. C. 

 Cliurch. ) IMouchot claimed, apparently correctly, that his engine was the first, 

 and Ericsson a<lmits that, " Some time previous to 1870, IMouchot made a small 

 model engine, a mere toy, actuated by steam generated on the plan of accumula- 

 lation by glass bells * * *. 



Ericsson gives full details of all his apparatus for determining the 

 solar constant in the record of his life's work, entitled, " Contribu- 

 tions to the Centennial Exhibition," New York, 1876; but unfortu- 

 nately he did not describe in detail therein the solar boilers, explain- 

 ing that " expei'ienced professional men will appreciate the motive, 

 viz, that of preventing enterprising persons from procuring patents 

 for modifications." He does, however, give us the following amount 

 of information : 



On grounds already fully explained, minute plans of my new system of ren- 

 dering sun power available for mechanical purposes will not be presented in 

 this work. The occasion, however, demands that I should present an outline 

 of the concentration apparatus before referred to. It consists of a series of 

 polished parabolic troughs, in combination with a system of metallic tubes 

 charged with water under pressure, exposed to the influence of converging solar 

 rays, the augmented molecular action produced by the concentration being 

 transferred to a central receiver, from which the accinniilated energy is com- 

 miinicated to a single motor. 



Thus the mechanical power developed by concentrated solar heat is imparted 

 to the solar steam engine without the intervention of a multitude of boilers, 

 glass bells, gauges, feeders, etc. Moreover, the concentration apparatus, imlike 

 the instrument of Mouchot, requires no parallactic motion, nor does its manage- 

 ment call for any kno^xledge of the sun's declination from day to day. Its 

 position is regulated by simply turning a handle until a certain index coincides 

 with a cei-tain bright line produced by the reflection of the sun's rays. 



His boilers seem to have been exceedingly eiRcient, for he claims that 

 " the mechanism which I have adopted for concentrating the sun's 

 radiant heat abstracts, on an average, during nine hours a day, for 

 all latitudes between the Equator and 45°, fully 3.5 units of heat per 



