100 
a maximum ; a result directly opposed to that derived from 
the Oxford observations, and to the conclusions drawn by 
Professor Tyndall from his experiments. If, however, we 
examine the conditions under which the observations were 
made at Greenwich, we find that while the ordinary thermo- 
meters on the shade stand were placed 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 
the greater or less prevalence of cloud or haze, and it is also 
evident that the amount of the latter will in general depend 
upon the degree of humidity of the air as determined from 
observations taken with the dry and wet bulb thermometers. 
Any alterations therefore which may take place in the calo- 
rific 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 maxi- 
mum temperatures in the sun and in the shade. The values of 
