48 



present in the air in the clearest and bluest skies. But it 

 is not known, as a fact, to what height this invisible vapour 

 attains in the air, for it is certainly present very far above 

 the highest ascents that aeronauts have made. 



Much attention has, of late years, been given to the 

 observations and recording of the hours of sunshine. Of 

 course in such records the remaining hours of the day 

 represent the time during which the sun's disc was not 

 visible on account of the presence of cloud or thick vapour. 

 Still these records have not been turned to any useful 

 account, nor do they reveal any hitherto unknown law 

 arising from the shining of the sun. 



Now there remains some unexplained cause for the 

 difference of the readings of two solar radiating thermo- 

 meters, one placed in vacuo, and the other exposed to the 

 direct action of the air in front of the sun, notwithstanding 

 the many theories or assumptions respecting it. It can 

 hardly be supposed that the decrease of solar radiation on 

 the surface of the earth is in con'^equence of the mere inability 

 of the solar radiating force to penetrate the cloud. It is 

 quite true, however, as has been noted, that in the presence 

 of cloud there is a decrement on the earth of direct solar 

 heat. But the question arises, what has become of the 

 apparently lost solar energy ? From our knowledge of the 

 effect of such force upon water it cannot be unreasonable to 

 assume that when the radiating solar force acts upon an inter- 

 vening cloud it will tend to its higher evaporation, and so 

 draw it upwards or disperse it into invisible vapour. This is 

 illustrated by the effect which a hot iron plate has upon 

 escaping steam. If a hot iron plate be placed amongst 

 escaping steam, it will be found that for a considerable 

 distance around the plate the steam is rendered invisible, 

 and being rendered hotter by the plate, it ascends higher in 

 the air than it otherwise would do, and re-appears as almost 

 invisible vapour a considerable distance from the plate. 



