94 abbot: solar constant of radiation 



form of instrument for measuring solar radiation. It was in- 

 vented about the year 1895. Two metal strips exactly similar 

 to one another, and blackened upon the front, are exposed 

 alternately to heating by the sun. Arrangement is provided for 

 passing an electrical current through the strip which is not at 

 the moment being heated by the sun. Thermo-elements, fas- 

 tened to the back of each strip, indicate when the temperature 

 of the exposed strip is equal to that of the strip which is electric- 

 ally heated. Under these circumstances it is assumed that the 

 energy of the electric current is equal to the energy received 

 from the sun. About 160 copies of this electrical compensation 

 pyrheliometer have been sent out from Upsala to different parts 

 of the world. 



Several other kinds of pyrheliometers have been used in re- 

 cent years, among them two forms which have been devised by 

 the writer. We shall have occasion to speak of these later. 



Early observations. Forbes observed with the Herschel actin- 

 ometer in 1832 at Brientzr and the Faulhorn. He showed that 

 the transmissibility of sun rays continually increases as the length 

 of path of the ray in air increases. Forbes rightly attributed this 

 to the non-homogeneity of the solar radiation, and the inequality 

 of transmission of the different component parts of it. Under 

 such circumstances Bouguer's formula of course cannot apply. 

 Forbes concluded that equal barometric columns of air give 

 equal transmission, whether taken from the high or low station. 

 In this he was wrong. He formed an empirical curve to repre- 

 sent all his observations at both stations, employing air masses 

 as abscissae and actinometer readings as ordinates. Instead of 

 extrapolating this curve directly to air mass zero he preferred to 

 find its tangents and thus derive the subsidiary curve of tangents 

 from which he derived a formula for extrapolating his observa- 

 tions. In this way he obtained results corresponding to the 

 value 2.85 calories per square centimeter per minute for the solar 

 constant. Thus Forbes cut loose entirely from Bouguer's ex- 

 ponential formula of atmospheric transmission. 



Pouillet observed in the years 1837 and 1838 at Paris. His 

 work was published before that of Forbes, although done later. 



