406 



Intelligence and Miscellaneous Articles. 



Let us first take the observations made with the sun at a height of 

 more than 60° above the horizon. 



On the 20th and 21st of July, the state of the atmosphere being 

 just that at which, as I have before said, the radiation at Geneva 

 was 15°' 5 of my actinometer, this number may be admitted for a 

 height of 400 metres above the sea-level*. On the 20th of July, at 

 the Glacier des Bossons, a height of 2500 metres, the thermometer of 

 the actinometer indicated 18°*63. On the 21st of July, at 11.30 a.m., 

 on the top of Mont Blanc, the thermometer of the actinometer marked 

 21°'13. These observations, being made under different atmospheric 

 pressures, should be reduced to what they would have been had the 

 thermometer been surrounded by air under a pressure of 760 millims. 

 To render the results more comparable, the atmospheric thickness 

 must in each case be calculated (product of the barometric height by 

 the secant of the zenith-distance of the sun). 



The following are the data : — 





Height. 



Pressure. 



Atmo- 

 spheric 

 thickness. 



Radiation. 



Cor- 

 rected. 



Uncor- 

 rected. 



Geneva 



metres. 



400 

 2500 

 4800 



millims. 

 730-0 

 5623 

 424-6 



805 

 645 

 473 



15-50 

 18-63 

 21-13 



15-34 

 17-32 

 18-62 



Glacier des Bossons. 

 Mont Blanc 



It follows from these numbers, first, that under the observed con- 

 ditions the ratio of the intensity of solar radiation on Mont Blanc to 

 that at Geneva is about f-. Thus the solar heat which has reached a 

 height of 4800 metres through the upper layers of the atmosphere 

 undergoes an absorption of i in traversing the lower layers of the 

 atmosphere to a height of 400 metres under an angle of 60° to 65°. 



It will be observed, in the second place, that the increase of the 

 radiation with the height is less rapid than the diminution of baro- 

 metric pressure, and than the diminution of the atmospheric thick- 

 ness. Taking the atmospheric thicknesses for abscissae, and the cor- 

 rected radiations as ordinates, three points are obtained representing 

 the above three observations. If a curve be drawn through these 

 three points, it will be seen to be concave towards the axis of the 

 abscissas \. This result is contrary to that which may be deduced 



* It may perhaps be thought that, instead of adopting this figure, it 

 would have been better to make simultaneous observations at the foot of 

 the mountain ; but, apart from the difficulty of this, I imagine measure- 

 ments made in a narrow valley like that of Chamouni present fewer gua- 

 rantees of accuracy. 



t By prolonging this curve indefinitely, the point where it cuts the 

 ordinate axis would correspond to the radiation for zero thickness. The 

 number thus obtained would be about 20 o, 5, and would represent the in- 



