-1859] WRITINGS OF JOSEPH HENRY. 151 



absorbed and another portion reflected from the atoms in its 

 passage through the air, and in the solution of the problem 

 under consideration it became necessary to know the amount 

 of loss from this cause. To ascertain this the experiment 

 was made while the sun was on the meridian and at different 

 degrees of elevation even down to near the horizon. The 

 diameter of the earth, the approximate height of the atmos- 

 phere, or the length of the column of air traversed b}' the 

 ray which passes from the zenith, and also the angle of 

 elevation of the sun being given, the lengths of the several 

 lines through the atmosphere traversed by the respective 

 rays were readily calculated; and if we suppose that the 

 amount of heat received at the outer limit of the atmosphere 

 is invariable it is not difficult to determine the part which 

 is absorbed. The numbers obtained by observation con- 

 sisted of two quantities, a constant and a variable one; the 

 former being the heat of the sun, and the latter the amount 

 absorbed in passing through the difierent lengths of atmos- 

 phere. 



From these data the amount of heat received from the sun 

 on a square centimetre at the limit of the atmosphere, (and 

 which it would equally receive at the surface of the earth if 

 the air did not absorb or reflect any of the incident rays,) was 

 ascertained to be 1"7633 units of heat in one minute of time: 

 equivalent to 11'376 units per square inch, in the same period. 

 It was also found that the atmospheric absorption of the rays 

 directly from the zenith was comprised between eighteen 

 and twenty -five-hundredths of the whole, even incases where 

 the sky was perfectly clear. 



The quantity of heat which the sun sends to one centi- 

 metre of the earth's surface during one minute of time by 

 its perpendicular action having been determined, it was not 

 difficult to ascertain the total quantity of heat received by the 

 whole illuminated hemisphere in the same time. Indeed this 

 quantit}'' is nearly the same as that which would fall on 

 the plane of a great circle of the earth. From this can 

 be readily deduced the amount of heat which would be dis- 

 tributed over the entire surface of the earth during a year; 



