262 RIETEOROLOGY AND ALLIED SUBJECTS. 



at least 1,700° C. The superficial area of the wire at this temperature 

 must have been 385 square millimeters, for which we will use 300 iu 

 order to take account of the cooling at both its ends. If, now, we com- 

 pute the quantity of heat iu calories radiated from this surface at this 

 temperature, according to the law of Dulong-Petit, assuming the radi- 

 ating power of polished platinum to be 0.00:3 according to La Provostaye 

 and Desaius, then we find for the whole surface 145.023 calories, whereas, 

 according to the computation of the electro-motive force, it was im- 

 possible that more than 42.3 calories could have been delivered. The 

 difference is enormous and is equally so iu other cases. {Z. 0. G. M., 

 XIV, p. 229.) 



Soret has investigated the absorption of heat by the earth's atmos- 

 phere, and the following review of his measures is given by Pictet. 

 The apparatus used by Soret is very similar to that subsequently used 

 by Violle in his investigations upon similar subjects. Moreover, 

 Violle selected the dynamic method and Soret the static method of ob- 

 servation. Soret finds that the variable absorption that our atmosphere 

 exerts upon the solar rays appears to be a consequence of the variation 

 iu the quantity of aqueous vapor, as well as the variable quantity of 

 dust, smoke, &c. All observations show plainly the influence of aqueous 

 vapor upon the absorption of the rays of heat, and other things being- 

 equal the absorption is greater in i)roportion as the quantity of aqueous 

 vapor is greater. Thus : 



(a) In winter, when the air is drier, the radiation is notably more in- 

 tense than in summer for an equal altitude of the sun. [Violle differs 

 from Soret iu this item, but Secchi agrees with the item.] 



(b) If we group the observations made with equal altitudes of the 

 sun, according to the degrees of humidity at the time of observation, 

 ■we find the intensity of radiation greater the di ier the air is. 



(c) Frequently a greater intensity of radiation is shown with dry air 

 than when with moister air the atmosphere is undeniably much purer 

 and more transparent. 



(d) The maxima of the intensities of the radiation correspond ordina- 

 rily, especially in summer, to exceptionally cold and dry days — lor in- 

 stance, during or immediately after prevailing strong north winds. 



The absorbing influence of suspended solid particles of organic and 

 inorganic matter is undoubted. This influence is felt over all the rays, 

 especially, however, over the most refractive. 



The absorbing power of the aqueous vapor is felt especially by the less 

 refractive rays. The annual maxima occur most frequently early in 

 spring, for in this time of year all the more favorable circumstances are 

 united — a considerable solar altitude, dry air, and a small quantity of 

 dust i^articles. 



The radiation is more intense for considerable altitudes than near 

 the horizon. The maximum of the day occurs a little before midday for 

 the high altitudes, while for lower altitAides it occurs somewhat after 

 midday. 



