ixx NOTES. 



C M ) p. 206. Adolphe Brongniart, in the Annales des Sciences natu- 

 relles, T. xv. p. 225. 



( 1M ) p. 207. Bischof, Warraelehre des inneren Erdkorpcrs, S. 324, 

 Anm. 2. 



(**>) p. 207. Homboldt, Asie centrale, T. i. p. 43. 

 (S 01 ) p. 208. On the theory of the isogeothermal lines (chthonisothermals;, 

 gee the ingenious discussions of Kupffer, in Poggend. Ann. Bd. xv. S. 184, 

 and Bd. xxxii. S. 270 ; in his Voyage dans 1'Oural, p. 382398 ; and in 

 the Edinburgh Journal of Sciences (New Series, vol. iv. p. 355). Compare 

 also Kamtz, Lehrb. der Meteor. Bd. ii. S. 217 ; and on the rising up of the 

 isogeotherrnals in mountainous countries, see Bischof, S. 174 198. 

 9 p- 208. Leopold von Buch, in Poggend. Ann. Bd. xii. S. 405. 

 f 3 ) p. 208. See my Rel. hist. T. ii. p. 22. The temperature of drops 

 of rain at Cumana was 22'3 C. (72'2 Fah.), when that of the air was 30 

 and 31 C. (86 to 88 Fah.) ; and during the fall of rain the temperature o 

 .the air sunk to 23'4 C. (about 74 Fah.) The initial temperature of the 

 drops depends on the height of the cloud in which they are formed, and on the 

 heat which its upper surface may have received from the direct effect of the 

 gun's rays ; but this temperature alters during their fall. The latent heat, set 

 free in their formation, causes them to have at first a temperature rather 

 above that of the surrounding medium ; and as they pass through lower and 

 warmer strata of air, this is somewhat raised, while at the same time their size 

 is increased by the condensation of the aqueous vapour contained in thos 

 strata. (Bischof, Warmelehre des inneren Erdkorpers, S. 73). The increase 

 of temperature from the cause described is, however, compensated by the loss 

 of heat which the drops undergo from evaporation from their surface. Apart 

 from the effect probably due to atmospheric electricity in thunder showers 

 the cooling influence of rain may be ascribed, first to the low initial tempera- 

 ture which the drops have acquired in the higher regions of the atmosphere, 

 and to the colder air of the higher strata which they bring down with them ; 

 and, lastly, to evaporation from the wet ground. This is the ordinary course 

 of the phenomenon; but in some rare instances (Humboldt, Rel. hist. T. iii. 

 p. 513) the drops are warmer than the lower strata of the atmosphere through 

 which they fall ? this may possibly be caused either by warm upper currents, 

 or by the heating of an extensive surface of thin cloud by the sun's rays. 

 Arago, in the Annuaire for 1836, p. 300, has shewn the connection 

 between the phenomenon of " supplementary rainbows," which are explained 

 by the interferences of luminous rays, and the size and increasing bulk of 



