PHYSICS. 107 



4. Radiation. 



Crova has communicated a memoir on the measurement 

 of the calorific intensity of solar radiations, and their absorp- 

 tion by the terrestrial atmosphere. The instruments used 

 were an actinometer of his own construction and a pyrheli- 

 ometer modified from that of Pouillet. He has observed an 

 annual variation of the intensity of solar radiation analogous 

 to the daily one, this intensity increasing rapidly from Janu- 

 ary to May, when it attains its maximum. He has observed 

 no relation between the values of this intensity and those 

 expressing the hygrometric state of the air. 



Crova has subsequently described the actinometer which 

 lie used to measure the calorific intensity of the solar radi- 

 ations and to determine their absorption by the terrestrial 

 atmosphere. It consists essentially of a large alcohol ther- 

 mometer, suitably incased, the bulb of which is exposed to 

 the direct rays of the sun. Observations carried on daily 

 and hourly enable the author to trace the curve representing 

 the calories received by each square centimeter per minute. 

 The differences observed in different hours of the day and 

 days of the year enable the calorific intensity to be calcu- 

 lated as a function of the thickness of the atmosphere trav- 

 ersed, and to calculate an approximate value for the solar 

 constant. Between 80 and 94 per cent, of the solar radi- 

 ations traverse unit thickness of the atmosphere. 



Haga has reopened the question of the absorbing power 

 for radiant heat by aqueous vapor, and shows by his experi- 

 ments that when columns of dry and moist air are allowed 

 to ascend in front of a thermo-pile arranged differentially 

 the effects are due to two causes: first, to the direct radi- 

 ation of the air column, which has been cooled by passing it 

 over moist pumice, or warmed by drying; and, second, to the 

 evaporation from the face of the pile caused by the dry air 

 which cools the pile, or to the condensation of moisture upon 

 the face by the moist air which warms it. These two causes 

 act in opposite directions; the second is the greater, and is 

 ternporaiy. 



Aymonnet has made at the Sorbonne, under the direction 

 of Desains, an examination of heat spectra, (1) to determine 

 the distribution of the heat in the calorific spectrum pro- 



