A T IM 



ATM 



as being the' tneiidian heiglits of t!ie fun, St tKe fiirnmer 

 and winter lolfticcs, in ti'.ij latitude of Cruli'ic in France. 

 Smith's Optics, Rem. 95. See Light, and Reflec- 

 Tio:;. 



Atmosphere, SoliiLriiy of ihr. See Eudiometry. 



Atmiisphbre, Teinpcra'.ure of the. The variable tempe- 

 rature of the atmofphere, at different feafons and in dilRrent 

 fituatioiis, has been the fr.bjesft of elaborate iiivelligation ; 

 and many fpsculations and theories have been propofed in 

 order to account for the changes which it undergoes. That 

 the prefence of the fun is the principal fource of heat as 

 well as of light, and its abfencc of cold, is too obvious to 

 have been ever doubted; and the effeft produced by the 

 greater or lefs obliquity of its rays has been long and tni!- 

 vtrfally obferveJ and acknowledged. From this f;ii£l, how- 

 ever, the ancient pkilofophers of Greece and Rome ico 

 hiilily inferred, that the torrid zone, under a vertical fun, 

 and the fiijrid zone, where its rays fall very obliquely, were 

 uninhabif.:ble. Time corrected this millake ; and prefented 

 rew phenomena which it has been found difficult to explain. 

 The hottcll days are frequently felt in the coldeil climates, 

 and tiie greateiV cold, as well as perpetual fnow, are found 

 ill countries bordering on, or even iuimedlately under the 

 equator. In the fame latitudes, very diil'crent temperatures 

 have bean obferved, not only in different, but even in the fame 

 liemilphere. The temperature of the caflern coalt. of North 

 Amenca differs widely from that of the wellern oppofite 

 coall of Europe, but agrees nearly with that of the ealfern 

 coaft of Afia lying between the fame parallels. Mem. 

 Philad. vol. i. Tlicfe, and fimilar circumllances, have made 

 it neceffary for meteorologifts to recur to other caufes of 

 varyinor temperature, bclides the immediate agency or ab- 

 fence of the fular rays. Dr. Halley has, indeed, proved, 

 that, abrtrafting from the intervention of fogs, miils, and 

 mountains of ice, the hotted weather, might in funimer, 

 take place even under the poles, the duration of the fun's 

 light more than compenfating for the obliquity of its direc- 

 tion (fee Heat) ; but as many pliylical caufes ob- 

 flrudl the adlivity of the folar rays in thefe and other 

 regions, it was llill neceffar)'- to recur to fonie other 

 canfe. At length M. De Mairan (Mem. Acad. Par. 1719 

 and 1767) difcovered, that the rigour of the cold of 

 winter is tempered by the heat imparted to the atmofphere 

 by the earth itftlf ; which heat, probablj' poffeffed from its 

 origin, ispreferved and renewed by the inceflant influences of 

 the fun, to which one half of its furface is conftantly expofcd. 

 Admitting this fact, the temperature of the atmofphere 

 mull depend on the capacity of the earth for receiving and 

 retaining heat, and for communicating it to the furrounding 

 medium. But as the earth is compoied of land and water, 

 it fliould be eonfidcred that the capacities of thefe con- 

 ftituent parts for receiving both heat and cold are very dif- 

 ferent. Land, particularly when dry, receives heat from 

 the fun's rays very readily, but tranfmits it through its 

 own fubftance to great depths very flowtly ; and on the 

 other hand, water, by reafon of its tranfparency, receives 

 heat ver)- flowly, but diffufes what it receives more readily. 

 Dr. Hales found, that in the month of Augufh 1724, when 

 the air, and the furface of the earth, were both at 88°, a 

 thermometer, placed. only t\yo inches under the furface, 

 ilood at 85°; another 16 inches under the furface indicated 

 70°, and a third 24 inches deep, ilood at 68°. The two 

 laft thermometevs preferved the fame temperature both day 

 and night, till the end of the month, and then fell to 63° 

 or 61°; the earth obflinatcly retaining its heat, at that 

 depth, though the temperature of the air freciuently varied. 

 €>n the 26lh of Odober, a thermometer, expofed to the 



air, flood at 3 J. 5' ; but one funk two inches in the earth 

 was heated to 43.85"; another at the depth of 16 inches 

 ffood at 4.3.8° ; a^.d another 24 inches deep, flievvcd 50° ; 

 and from the ill to the 12th of November, when, the tem- 

 perature of the external air was 27°, a thermometer placed 

 at the depth of 24 inches flood at 43.8°; but from the 

 month of March to that of Septem.ber in the following 

 year, the external air was conftantly warmer than the earth 

 at the depth of 16 inches or 2 feet; the feafon, however, 

 was very rainy, and the evaporation, thus occalioned, pre- 

 vented the earth from being warmed fo much as it otherwife 

 might have been. Hales Veget. Statics, vol. i. p. 61, &c. 

 From thefe experimerits it may be interred, that the furface 

 of the earth is much heated during the fummer, but that 

 the heat defcends very fiuwly, a great part of it being com- 

 municated to the air ; that during the winter, the earth 

 gives out to the air the heat whicli it had received during 

 the fummer ; and that \Vct fumm.ers mull be fucceeded by 

 cold vt-r.:terB. The experiments of Dr. Hale- furnifli nearly 

 the fame refults with tliofe of Mariotte (Sur le Froid aud 

 le Chaud, p. 189.) ; who found, that the earth is gradually 

 heated during the fummer, and as gradually cooled duriiig 

 the winter months ; and that, at the diltance of a few feet 

 under the furface, it is conilantiy warmer tiian the external 

 air; and the excels was found to remain till April, when 

 the furface is again heated by the fun's rays, and flov.-ly 

 tranfmits its heat downwards.' Hence it, appears, that at 

 the diilance of about 80. or 90 feet below the furface, pro- 

 vided that there be a communication with the external air, 

 or at a lefs depth if there be no fuch communication, the 

 temperature of the earth admits of very flight variation, and 

 generally approaches to the mean annual heat. Then the 

 temperature of fpring is nearly the fame as the annual 

 temperature, and varies very little. M. Van Swinden has 

 obferveel, that the greatell cold, and even that which ex- 

 ceeds o of Fahrenheit's fcale, if it lails no more than a fewr 

 days, penetrates no deeper than 20 inches when the earth 

 is covered with fnow, and not above 10 inches if no fnow lies 

 on tlie furface ; and this fa6l evinces the important and ufe- 

 ful purpofes anfwered by this covering in high northern 

 latitudes. Such fafts tend to prove, that the heat of the 

 earth does not increafe as we defctnd into it ; but at the 

 greatell depths it is nearly the fame as the mean annual 

 temperature of the latitude. It has been obferved, that 

 land is capable of receiving much more heat or cold than 

 water. To this purpofe, Dr. Raymond found, in the neigh- 

 bourhood of Marleilles, land frequently heated to 160°; 

 but he never found the fea hotter than 77° ; and in win- 

 ter he frequently obferved the earth cooled down to 14° 

 or 15°, but the fea never lower than 44° or 45°. (Mem. de 

 la Societ. de Med. de Paris, an. 1778. p. 70.) From thefe 

 facls it is an obvious inference, that the atmofphere which 

 lies over the fea Ihould maintain a more uniform temperature 

 than that over the land ; and this is found to be the faft ; 

 nor is it difficult to give a fatisfaclory explication of it. 

 During fummer, the temperature of the fea on its furface is 

 conflantly diminilhed by the procefs of evaporation ; and 

 in the winter, when the fupcriicial water is cooled, it de- 

 fcends by its augmented gravity to the bottom, and its 

 place is occupied by water of a higher temperature. This 

 alternate change of this heavier and lighter air proceeds, 

 and the winter elapfes before the atmofphere has dimi.iilhed 

 the temperature of the water below a certain degree. Be- 

 tween the mean annual temperature of the atmofphere over 

 the ocean, and that of countries fituatcd at a coniidcrable 

 diilance irom it, there is a very perceptible difference. As 

 the fea is never heated to the fame degree as the land, 



the 



