APPENDIX TO MEMOIR OF PELTIER. 197 



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Tlie succession of electrical signs is accordingly as follows : the earth is res- 

 inous ; the inferior vapors which give rise to the white mists ai"e vitreous ; tho 

 middle vapors, which produce by their condensation the large, slate-gray strata, 

 are resinous ; the superior vapors, which in their turn generate the volaminoua 

 white cumulus, are vitreous ; and, finally, the tropical current which overtops 

 the whole is again resinoiis, like the earth. 



In a medium so movable as the air, and subject, moreover, to so many differ- 

 ent influences, this triple distribution has stability only for. the succession of 

 vapois in time and space, and not for those which, at a given moment, form the 

 triple superposition. We have before said that the inferior vapors retained but 

 for a short time their vitreous electricity, and that they quickly resolved them- 

 selves into drizzling rain ; but that is not all. When, through the lowering of 

 the temperature, the vapors have ceased to ascend and to thus feed the superior 

 zone, the cumulus which is suspended therein disappears by degrees under the 

 form of elastic vapors. This new transformation is effected so much the more 

 rajiidly as the air at that altitude is drier, and as the electric action of the trop- 

 ical current is more intense. 



The inferior vitreous vapors, those which presented themselves under the fonn 

 of mist or fog, being resolved into drizzle or dew, the visible, superior, vitreous 

 vapors, those which presented themselves under the form of cunmlus, having 

 returned to the state of elastic vapors, there remains in the atmosphere nothing 

 but the intermediate vapors, which ai'e alone seen at evening and at night, 

 extended in long, ojiaque curtains, forming clouds of a slaty gray. SuiToundecl 

 by an electricity of the same natui'e as the globe and the tropical current, the 

 repulsion which these clouds encounter on each side retards their re- vaporization ; 

 they repass into the state of elastic vapors only when, notwithstanding the 

 re- vaporization of the white and vitreous clouds, the air is still far from satura- 

 tion ; they change their state by the sole force of hygrometric affinity, and not 

 by the help of electric attractions, as takes place in regard to the two other 

 zones. Hence it often happens that we still see, the next morning, portions of 

 these graj'ish strata not re-vaporized, and which present themselves under the 

 form of dark spots, or even extensive black bands, strongly relieved by their 

 deep color in the midst of the ruddy hues of the dawn. 



On the influence of hydro-meteors on the distribution of temperature at the sur- 

 face of the ground. — The temperature of a place depends not alone on its lati- 

 tude ,- it depends also on its longitude. Thus Eastport, in America, and Stock- 

 holm, in Sweden, have a mean temparature of about 5°.o, and yet their latitude 

 differs by 14 degrees. New York and Naples are in the same latitude, but the 

 mean temperature of winter at Naples is 9°.9, while that of New York is — 1°.20, 

 the difference being 11.1 degrees. 



By uniting by lines all the points for which the mean temperature is the same, 

 we obtain curves which Humboldt first traced on maps, and which are desig- 

 nated under the name of isothermal curves. These lines are very far from furm- 

 ing parallels with the equator : thus the isothermal line of 10° passes successively 

 by Fort George, (10°.l); bv Erasmus Hall, near New York, (10°. 7); bv Dublin, 

 (9°.56) ; bv London, (10.4) • byHarlem,(10°.0); andbyOdessa, (9°.8G);' that is to 

 say, by 46°.18 of north latitude; 40°.37 ; 53°.21; 5i°.31 ; 52°.23; and 4G°.2S. 

 The extent of the divergence is therefore about 13°. From this Ave see that the 

 angle under which the rays of the sun strike the earth is not the only element 

 which determines the temperature of a place. Several other causes, in effect, 

 contribute then* action. 



The trade winds impelling towards the equator masses of air proceeding from 

 high latitudes, refresh the intertropical regions. On the contrary, the warm 

 wind of the southwest, which proceeds from the equator and which sinks 

 towards the earth in proportion as it advances nearer the poles, communicates to 

 the regions which it touches a portion of its heat and moderates the rigor of 



