Intelligence and Miscellaneous Articles. 405 



dications of the instrument may thus be corrected by calculation, 

 and reduced to what they would have been if the thermometer had 

 been surrounded by air under a pressure of 760 millims. 



I think I may assume that, when this correction is made, the num- 

 ber of degrees above zero indicated by the thermometer gives a very 

 approximate measure of the intensity of the direct solar radiation. 



Observations made at Geneva. — A series of observations made 

 at Geneva has shown me that the moisture of the atmosphere influ- 

 ences the direct solar radiation ; in general, the more aqueous vapour 

 there is in the air the less is the radiation. I will limit myself to 

 adducing some examples. 



Comparing the small number of observations which I have been 

 able to make in winter with those of summer, we see that with equal 

 radiation the height of the sun above the horizon was considerably 

 greater in summer than in winter. 



On several occasions a stronger radiation has been observed during 

 dry than during moist weather, although the atmosphere was un- 

 doubtedly purer and more transparent in the second case than in the 

 first. The presence, then, of solid or of liquid particles diffused in 

 the air is insufficient to account for variations in the solar radia- 

 tion. 



With virtually equal solar heights the minima of radiation corre- 

 spond to days on which the tension of aqueous vapour was strongest, 

 and the maxima of radiation to the dry est weather (during or imme- 

 diately after strong north winds) . Thus, for solar heights greater than 

 60°, the minimum temperature was 14°*82 on the 2nd of last June, 

 the tension of aqueous vapour being 14 millims., after a series of moist 

 days : it indicated a maximum of 15 0, 93 on the 10th of July, on the 

 day after a strong north wind during which the temperature sank 

 to 7°, and after a series of exceptionally dry and cold days. 



Yet as these variations are not great, while fog exerts also an 

 undoubted influence, it would be very interesting to have a long 

 series of observations, the means of which would enable us to de- 

 termine the exact relation between the moisture of the atmosphere 

 and the intensity of the direct solar radiation. 



The radiation is remarkably constant while the sun is at a great 

 height above the horizon, and when the atmospheric conditions remain 

 almost the same. Thus observations made between 11.30 a.m. and 

 1.30 p.m., the sun being more than 60° above the horizon, the sky 

 clear, the tension of aqueous vapour 8 to 11 millims., excepting the 

 case of strong north winds, have regularly given radiations comprised 

 between 15 0, 31 and 15°'59. It may thus be assumed that the num- 

 ber 15°'5 represents the solar radiation of Geneva under these cir- 

 cumstances. 



Observations at different heights. — The frequency of clouds on 

 mountains renders it difficult to make observations at different 

 heights ; and thus I have only been able to make an inconsiderable 

 number of them. Yet particularly in an ascent of Mont Blanc, on 

 the 20th and 21st of July last, during exceptionally fine weather, I 

 obtained results which, if I am not deceived, possess some interest. 



