98 



wind blowing past C and the graduated stem D, which are shaded 

 from the sunshine, or which may be kept immersed in melting ice. 

 although this is not done at Montsouris. In order that the record of 

 liquid condensed in C and D may be proportional to the heat received 

 by A it is necessary that the volume of condensed liquid be inde-' 

 pendent of the temperature of the air and of the volume of liquid 

 i-emaining in the bulb A, and be controlled only by the excess of 

 radiant heat received by A over that received by C and D. 



The comparisons thsit have been made at Montsouris between this 

 Bellani radiometer and the Marie-Davy actinometer, or the so-called 

 conjugate thermometers, show that the Bellani apparatus does not 

 perfectly satisfy the theoretical conditions, but as it is extremely 

 sensitive, since it distills 16 or 17 centimeters daily, and as the appa- 

 ratus is not costly, Descroix has devised a formula for reducing its 

 results to a standard. The following table gives the results of actual 

 observations, showing the average results for the middle portion of 

 perfectly clear days "at Montsouris, near Paris, monthly during the 

 growing seasons of 1885 and 1886. The column N shows the number 

 of clear days; V, the volume of alcohol distilled over from 9 a. m. 

 to 3 p. m. on these days in the Bellani apparatus; t, the average 

 temperature of the air in the shade; Tj, the average temperature 

 of the black bulb thermometer in vacuum in the sunshine; T., aver- 

 age temperature of the bright-bulb thermometer in vacuum in the 

 sunshine; T^ — To is the average difference of these conjugate ther- 

 mometers at midday; R, total illumination from the sky at midday, 

 expressed in Marie-Davy's actinometric percentages or degrees. 



Comparison of actinometric results for perfectly clear days at Montsouris. near 

 Paris, by the Bellani radiometer and by the Arago-Davy actinometer. 



Notwithstanding the simplicity of Bellani's apparatus and the 

 ingenuity of the idea embodied therein it is evident that it needs an 

 important modification, viz, the evaporation and condensation should 



