150 



MR. JOSEPH BAXENDELL 



with their bulbs about 4 feet from the ground, the solar- 

 radiation thermometer was placed in an open box about 

 13 inches high_, with its bulb about 10 inches above the 

 bottom of the box. Now a little consideration will show 

 that on a clear calm day in summer the air in this box 

 will be heated to a temperature several degrees above that 

 of the air at 4 feet from the ground ; while on the other 

 hand in winter it will often be several degrees colder. 

 The readings of a thermometer placed in it will therefore 

 be too high in summer and too low in winter; and the 

 magnitude of the errors may well be sufficient not only to 

 mask the true action of the varying amount of the aqueous 

 vapour in the atmosphere, but to lead to conclusions directly 

 at variance with the truth. 



The heating effect of the sun^s rays at the surface of the 

 earth during a given interval obviously depends upon the 

 greater or less prevalence of cloud or haze, and it is also 

 evident that the amount of the latter will in general de- 

 pend upon the degree of humidity of the air as deter- 

 mined from observations taken with the dry- and wet-bulb 

 thermometers. Any alterations, therefore, which may 

 take place in the calorific intensity of the sun^s light ought 

 to be indicated by a comparison of the differences between 

 the mean temperatures of the air and of evaporation, and 

 those between the maximum temperatures in the sun and 

 in the shade. The values of these elements for the years 

 1859-64 at Oxford, and their ratios, are as follows : — 





Maximum 



in sun less 



maximum 



in shade. 



Temperature 



of air les9 



temperature 



of evaporation. 



Eatio. 



i8<;q 



12-85 

 10-78 

 11-24 



9-37 

 9-78 

 981 



2*67 

 1-98 

 2-49 



2-72 

 2-87 

 2-73 



4'8l 

 5 "44 

 4-51 

 3 '44 

 3-40 

 359 



i860 



1861 



1862 



1867 



1864. 





