90 



As those who can not make use of the actinometrie degrees deduced 

 by Marie-Davy from his observations of his conjugate bulbs will 

 necessarily have to use either the simple observations of clear sky 

 and cloudy sky, as given by the sunshine recorder, or the equivalent 

 personal observations of the clouds, I give the following tables, which 

 show how nearly parallel these two phenomena may be. Evidently 

 in our study of the influence of insolation on crops in America from 

 year to year w^e may use the sunshine recorder or the ratio between 

 the actual and the maximum possible duration of sunshine without 

 much error, at least in the growing season. 



Mean of fire daily actinometrie observations at Montsouris, expressed in Mari/'- 

 Davy's actinometrie degrees or percentages of maximum possible intensity. 



Month. 



1875. 1876. 18' 



April 



May 



June -. -.. 



July 



August 



September 



Average 



44.1 40.1 

 47.7 



46.0 48.8 



47.3 52.1 



39.9 42.0 



35.7 30.9 



43.5 43.5 



36.3 

 38.7 

 54.5 

 48.6 

 43.2 

 31.4 



42.1 



35.4 

 41.5 



47.7 

 50.6 

 37.8 

 30.9 



40.7 



40.6 

 45.1 

 41.2 

 42.3 

 32.7 



38.0 



38.9 

 50.3 

 41.2 

 50.0 



33.0 

 48.9 



52.0 

 40.3 



39.7 

 47.4 

 47.0 

 46.6 

 34.0 

 27.1 



40.3 



Mean of five daily observations of the cloudiness at Montsouris expressed as ratio 

 of the actual duration of sunshine to the maximum possible duration. 



RELATIVE TOTAL HEAT RECEIVED FROM SUN AND SKY 

 DURING ANY DAY, BY HORIZONTAL SURFACES. 



A more accurate way of considering the amount of insolation at 

 any locality is' to compute the total radiation (expressed by its equiva- 

 lent heat in calories) received by a horizontal surface in the natural 

 daytime of that day and latitude, taking account of the absorption by 

 the atmosphere. (See Annales Agronomique, 1878, IV, pp. 270-29(5, 

 or Ann. Report Chief Signal Officer for 1881, pp. 1200-121G.) This 

 has been done by Aymonnet by a graphic method. He assumes that 

 if the sun were in the zenith then the unit of horizontal earth's sur- 

 face would, because of atmospheric absorption, receive only 0.75 of 



