96 



Further details of Weber's results are given in the German periodical, 

 Photographische Mitteilungen, edited bv Professor Vogel, at Berlin. 

 It is worth while to call attention to the fact that during the long 

 twilights of northern latitudes in midsummer plants receive an appre- 

 ciable quantity of the blue radiations from the sky, while receiving 

 little or nothing of the red, or heat, rays. 



M ARCH AND' S SELF-REGISTERING CHEMICAL ACTINOMETER. 



A convenient form of registering actinometer is that devised by 

 Marchand (1875), which he at first called '' photantitupimeter," but 

 wdiich name he afterwards contracted and modified to '" phantupi- 

 meter.*"' This consists of a vertical graduated tube, closed at the 

 upper end, into which there can escape and be measured the carbonic 

 acid gas given off by the decomposition of a mixture of solutions of 

 perchloride of iron and oxalic acid. By the action of sunshine on 

 this mixture, carbonic acid gas is slowly disengaged, and by its accumu- 

 lation in the measuring tube gives us apparently a means of deter- 

 mining the sum total of the influences of the sun during any period. 

 This apparatus was diligently employed for many years by Marchand 

 at Fecamp, near Havre, and has afforded him many interesting 

 results. 



COMPARISON OF MARCHAND' S AND MARIE DAVY'S RESULTS. 



Radau (1877), in his work on Light and Climate, states that the 

 results given by different methods of measurement of sunshine appear 

 to differ largely among themselves, but yet there is a certain simi- 

 larity in the figures. The accompanying table shows the results of 

 observations by Marchand's chemical method and by Marie-Davy's 

 thermometric method, or conjugate thermometers, which latter, on 

 account of its convenience, has been widely adopted. 



If the atmosphere were not so very different at these two localities, 

 we could have hoped to use the monthly ratios of these numbers for 



