Solar Radiation 397 



comme temperature apparente au centre du soleil, en em- 

 ployant le coefficient de transmission zenithale 0*91 auquel 

 1'avaient conduit ses nombreuses mesures. Le telescope 

 de mon systeme dont il se servait fut reetalonne apres ces 

 mesures au National Physical Laboratory de Teddington.... 

 Pointe a Teddington, voisin du niveau de la mer, par un 

 jour beau et tres sec, sur le soleil, cet appareil a fourni 

 comme moyenne de huit mesures tres concordantes : 5153 

 comme temperature de 1'astre. 



" Le coefficient zenithal serait pour cette station 



L'absorption ne serait done que de 26 pour 100. 



"Ceci ne doit pas trop etonner puisque le maximum de 

 1'energie solaire est dans la partie lumineuse du spectre, ou 

 la vapeur d'eau produit son minimum d'effet." 



We see then that in the space of ten years the value of 

 the solar constant as determined by indirect methods has 

 fallen from 4/0 to 17 owing to the gradual revelation of 

 their defects. 



The solar calorimeter which I constructed and used first 

 in 1882 furnishes a direct measure of the sun's heating power 

 by the determination of the mass of water which it can con- 

 vert into steam. // furnishes one simple equation for the 

 determination of one unknoivn quantity. 



Since I used it on the banks of the Nile, very little above 

 the level of the sea, it has been used by Mr Michie Smith 

 at the high-level observatory of Kodaikanal attached to that 

 of Madras. 



The first object, in whatever locality it may be exposed, 

 is to obtain the maximum heating effect which the sun 

 can produce there under the most favourable circumstances. 

 During the few days in May 1882, when I used the instru- 

 ment at Sohag, the sun attained a meridian altitude of 

 roughly 83, so that its power differed very little from that 

 of a vertical sun. The latitude of Kodaikanal is 10 14' X., 

 and there the meridian altitude of the sun is greater than 80' 



