AND THE ACTINIC CONSTITUTION OF TIIK ATMOSPHERE. 



13 



These two months of uoiuteirupted obsei'vatiou [n-ove alfeady tliat the solar 

 combustiou passes through very different values within 24 hours. These changes 

 are sometimes very sudden and exceed especially those of the thermometer, the 

 barometer, and even those of the average brightness of the day. The actinometric 

 effect does not show, therefore, that appro.ximate constancy, Avhich makes it rela- 

 tively so easy to measure the other eft'ects of solar radiation ; it requires a very 

 close and minute iuvesti2;ation. 



While it amounts to little or nothing at all in overcast and rainy weather, it 

 rises very perceptibly during fine, sunny days; but it seems to be sul)ject to other 

 influences yet beside those which we have mentioned when we spoke of " a fine 

 day," " fine weather," etc. If we find, in fact, that the days from the 20th to the 

 24th Septembei- resemble each other very closely, tis far as their external physi- 

 ognomy is concerned, and are also very much alike in })<>iiit of actinometiy, we 

 have on the other hand the example of October 17th, 18th, and 19th, during 

 which the degree of combustion was the same, and this although the weather had 

 been very fine during the first two days and very indifferent during the last. Au 

 instance of the opposite nature is offered to us by the Gth, 7th, and 8th of Septem- 

 ber, which differed very much in their actinometric aspect, whilst they resembled 

 each other so far as their external physiognomy was concerned. 



It would be interestini? to find out under what influences these variations are 

 produced. In the meantime, until we reach that point, let us notice that the com- 

 bustion on the finest days in October does not amount to as much as that obtained 

 on the finest days in September, and that the latter again do not equal the fine 



