49 



This being so, it will happen that if the solar radiating force 

 become stronger at one time than another, at sucli periods 

 the deeper or higher, and rarer will become the amount of 

 the invisible vapour of the air. And this will be so, not- 

 withstanding the fact that atmospheric vapour is mostly 

 condensed and precipitated as rain. 



Now it happens that the more the daylight has been free 

 from cloud or dense vapour as thick haze or fog, the greater 

 has been the difference of the reading between a solar 

 radiating thermometer exposed before the sun in free air and 

 one mounted in vacuo. That I might, as far as possible, be 

 satisfied, that this difference was due almost entirely to 

 skies free from cloud during the sun's presence, I observed 

 carefully the readings of the two thermometers on days 

 during which I was certain the sun's disc was never seen at 

 my station at all. And after a correction which I instituted 

 and applied to the readings of tlie thermometer in vacuo, 

 and which I deemed sufficient to account for the differ- 

 ence of temperature recorded by it in the presence of cloud, 

 I considered both thermometers as reading from a common 

 Zero. After this I found that the readings of the two 

 thermometers, the correction being applied, were closer 

 according to the kind of blueness of the sky through which 

 the sun shone. If the blue were tinged with a kind of grey, 

 and the more it was so tinged, tlie nearer the readings of the 

 two thermometers were to each other, and the bluer the sky 

 the wider were the readings apart. The durations of sun- 

 shine did not affect the difference to any great extent, unless 

 they were very short periods. Hence I arrived at the con- 

 clusion that the difference of the reading of the two ther- 

 mometers indicated an evaporating power of the sun upon 

 the cloud or vapour in the air, thereby causing it to become 

 rarer, and more and more invisible, and so to rise from the 

 lower to the higher regions of the atmosphere. As the air 

 thus became clearer solar radiation on the surface of the 



