Intelligence and Miscellaneous Articles. 239 



4. Observing in summer near the horizon, at an elevation of 30 

 to 34 degrees, the temperature rises merely to 6°' 5. 



5. The rapidity with which the blackened thermometer mounts 

 is scarcely different in summer and winter up to 10 or 11 degrees; 

 but after this limit the maximum is sooner obtained in summer than 

 in winter. 



The results obtained in the latter season were quite unexpected ; 

 for I believed I should have in winter, observing with a sun's height 

 of about 28 degrees, a temperature at most equal to what I had in 

 summer at 32 degrees of elevation, for the atmospheric height was 

 almost the same ; but it was not so. In the meridian, I obtained 

 almost the same value as in summer, although the rays traversed 

 double the thickness of atmosphere, while this double thickness in 

 summer diminishes the force of radiation and reduces it to the half. 



These phenomena would be inexplicable if the absorbing power of 

 aqueous vapour were not known*. In fact, if we compare observa-- 

 tions made at the same height in summer and in winter, we find the 

 radiation of summer one-half, even though the atmospheric thickness 

 is the same ; but aqueous vapour, which in summer in our climate 

 has a tension of 13 to 14 millims., in winter has only 7 to 8. It is 

 difficult to estimate the absolute quantity of vapour which exists in 

 the path of the rays, for the psychrometer only gives the quantity 

 near the ground, which differs materially from that above. Moreover, 

 the level of vapour is in summer much higher than in winter. Yet 

 it would not be far from the truth to admit that in summer there is 

 at least double as much as in winter. Thus we find that with an 

 equal height the radiation is reduced to one-half in summer, as the 

 quantity of vapour requires. 



Two interesting conclusions follow from these facts : — (1) That in 

 summer the radiation in the meridian is greatly enfeebled by aqueous 

 vapour, so that without it we should have double the radiation (it 

 would be necessary to verify this for high mountains, which would 

 be well worth the trouble). (2) That the absorbing force of vapour 

 is very great ; for we see that a quantity of 7 to 8 millims. in excess 

 in summer produces an absorption equal to that of the thickness of 

 the entire atmosphere of gas in winter, which would give for the 

 absorbing force of vapour 95 times that of air. In these calculations 

 we have not allowed for the greater elevation of the vaporous layer 

 in summer, nor for the phenomenon of thermochrose, which causes 

 the first layers to absorb more than the following, But in general 

 we may admit that Mr. Tyndall's result, which makes it sixty times 

 as much as air, is not far from the truth. ■ 



My researches on this important subject are not finished ; I intend 

 to improve the apparatus and observe in different seasons with 

 greater care ; but the first results have been so interesting that, 

 although merely sketched, I thought it right to present them to the 

 Academy. Further researches may elucidate the numerous ques- 

 tions which attach themselves to this subject, and may separate that 



* In winter the sun's radiation is stronger because he is nearer us, but 

 that does not produce a difference of more than -fa. 



