236 Prof. Minchin's Experiments in Photoelectricity. 



energy, the steam-engine. I usually find two objections 

 made against any investigation in this direction. The first 

 is, that the attempt is contrary to the principle of the conser- 

 vation of energy. The objection, however, does not seem to 

 be well founded ; because it is conceivable that a photo- 

 electric battery may yet be found which will simply act as a 

 transformer of the energy which it receives from the sun, 

 while its own materials, being merely the implements used 

 in the process, may be almost wholly unmodified. The 

 energy thus taken out of the sun may finally be radiated 

 out into space from the earth in the form of heat, if it is true 

 that all forms of energy must ultimately pass into this form 

 — a proposition which, being a very wide generalization 

 from our experience on the earth, it may be permitted to 

 doubt in the universal necessity claimed for it. 



The second objection is that there is not energy enough in 

 the solar rays at the distance of the earth to supply the work 

 desired. This objection is founded on the experiments of 

 Pouillet, Violle, and others, who have estimated the solar 

 energy incident per square foot per second on the surface of 

 the earth. Let us see how much energy, according to the 

 assumptions based on these experiments, is at our disposal. 



The quantity of solar energy, assumed to be measured in 

 metric thermal units, which is incident, normally, every 

 minute on a square centimetre at the distance of the earth is 

 is given by the expression 



A a% 



where, according to Pouillet, A (the solar constant) is 

 1*7633 calories, a (the atmospheric constant) is something 

 between -7244 and '7888 — let us say that a=*75 ; and e is 

 the ratio of the thickness of atmosphere tvaversed to the 

 normal thickness of the atmosphere measured from the place 

 of observation. M. Violle employs a formula of this form, 

 but, according to him, e involves the height of the barometer 

 and the pressure of aqueous vapour present in the air, and 

 A = 2*54 calories. At the superior limit of the atmosphere 

 e is, of course, zero, and at the surface of the earth, for 

 normal rays, 6=1. Taking this latter value of e, and con- 

 verting the thermal units into ergs, the quantity of energy 

 incident per square centimetre per minute is 



1-3224 x 42 x 10 6 ergs 



according to Pouillet. and 



1-905 x 42 xlO 6 ergs 



