A T U 



die mean temperature of funimer over the Tea may be con- 

 fidered as lower than that over the land. In winter, when 

 the force of the fun's rays is weakened, the fea imparts its 

 heat to the atmofphere much more readily than the earth 

 The mean temperature on fea, is, therefore, at tliis fcafon 

 higlier than on land, and in cold eountrics this difference in 

 the evolution of heat is fo very confidtrable, that it more 

 than counterbalances the difference which takes place in 

 fummer; infomuch that in high latitudes, the mean annual 

 temperature at fea ought to exceed that on the land. Mr. 

 Kirwan obferves, that, in order to find the temperature in 

 any place, fituate between the latitudes 70-" and 35°, the 

 Itandard temperature for the fame latitude fiiould be lowered 

 ^d of a degree for every 50 miles of dillance ; fince in 

 winter the cold always increafes in proportion to the di- 

 llance from the ftandard. At a kfs diftance than 50 miles 

 the atmofphere on the ocean and land are fo blended toge- 

 ther by the agency of fea and land winds, that little dif- 

 ference is perceptible in the annual mean temperature. 

 In lower latitudes than 30°, the folar rays even in winter 

 aft with . no inconfiderable force, the furface of the earth 

 alio retains a pretty confiderable degree of heat, and con 

 fequently the mean annual tem^peratures of the fea and land 

 prelerve a greater equality. In proportion as we approach 

 to the equator, the force of the fan's rays in winter acts 

 ■with additional energy, and the mean temperature of the 

 land atmofphere at this feafon approximates nearer and 

 nearer to that of the fea, till at the equator they become 

 equal. 



In latitudes diftant from the equator, idands are warmer 

 than continents, becaufe they participate more of the tem- 

 perature of the fea. Countries that lie fouthward of any 

 fea, are warmer than thofe that have the fame fea to the 

 fouth of them, at leail in our hemifphere, becaufe the 

 winds that fhould cool them in winter are tempered, by 

 paffing to them from that fea ; and thofe that are northward 

 of the fea are cooled in fummer by the breezes that iffue 

 from it ; but a northern or fouthern bearing of the fea ren- 

 ders a country wanner, than if it lay either to the call or 

 weft. Trafts of land which are covered with trees and lux- 

 uriant vegetables, are much colder than thofe which have 

 lefs furface of vegetable matter: for though living vegetables 

 alter their temperature flowly, and with difficulty, yet the 

 evaporation from their numerous furfaces is much greateV 

 than from the fame fpace of land uncovered with vegetables ; 

 and befides, when they are tall and clofe, as forelts, they 

 exclude the fun's rays, and (belter the winter fnows from the 

 wind and fun. From fome experiments of Mr. Williams 

 (Philad. Tranf. vol. ii. p. 150.) it appears, that forefts dif- 

 charge one-third more vapour into the atmofphere, than the 

 fame fpace of ground would do if actually covered with 

 water. From this reafoning it appears, that woody coun- 

 tries are much colder than thofe that are open and culti- 

 vated ; and it will enable us to account for the amelioration 

 of climate that attends agricultural cultivation. See Cli- 

 mate. 



Another principal fource of heat, befides the fun's rays 

 and earth, which may be regarded as a repofitory of heat, 

 is the condenfation of vapour. It is well known, that va- 

 pour contains a quantity of the matter of heat, which pro- 

 educes no other effed but that of making it alfume an aerial 

 expanded ftate, until the vapour is condenfcd into a liquid , 

 but during this condenfation a quantity of feniible heat is 

 let loofe, which warms the fuiTounding atmofphere. This 

 condenfation is frequently occai'ioned by the attraction of an 

 eleftrical cloud ; and hence proceeds the fultrinefs which we 

 often experience before rain. 



Vol. III. 



ATM 



Notwithdanding the variations of temperatare that occur 

 in evci7 climate, and at every feafon, there is a mean tern- 

 peratnre from which the atmofphere fcldom deviates beyond 

 11^"^"^ """'''*"' °^ <'<^g''<^'s. In order to determine this 

 Mr. PlRyfair, profeflbrol mathematics in the univerllty of 

 Edinburgh (lee Edinb Tranf. vol. v. part 2. for i8c2, 

 p. 193.), divides every month, into three parts, and exhibit* 

 the Rate of the barometer and thennonietcr for each cf 

 thefe divifions. In his tables the three tirft columns contain 

 the greateft, Icaft, and mean height of the barometer ; and 

 the fourtli column gives the temperature of the air in the 

 room where the barometer is kept. The fifth and 6th co- 

 lumns fhcw the grcatell height of the thermometer in the 

 air, obftrved during the ten days to which the numbers refer; 

 the next three give the mean heights, ;is obferved at three 

 different times every day ; viz. at eight m the morning, ten 

 in the evening, and as nearly as poffible to the warmed time 

 of the day, or fome time between mid-day and three in the 

 afternoon. The mean of all thefe is taken for the mean 

 temperature of the day, which being computed for each 

 day, the mean of all thefe mean temperatures is fet down 

 for the mean temperature of the atmofphere for every one 

 of the thirty-fix divifions of the year. The mean of the 

 three divifions of every month is given in the next column, 

 under the title of the mean temperature of the month. 

 It is prefumed, fays Mr. Playfair, that the mean tempera- 

 tures, which are the points moff; difficult to be afccnaincd, 

 are given with tolerable exaflnefs, as tliey are deduced from 

 three obfervations made every day, of which the firil, viz. 

 that at eight in the morning, is not far from the medium 

 tempertture of the whole day, and the other two are as 

 near as circumftances will allow, to the two extremes of 

 greateft heat and greateft cold. At Edinburgh, the mean 

 temperature for the year 1 797 was 48.04° ; for 1 798, 49.28'' ; 

 and for 1799, 46.13°. From a mean of the obfervations 

 made at the houfe of the Royal Society, from 1772 to 1 780, 

 the annual temperature of London appears to be 51. 9", or 

 in round numbers 52°. 



The greateft mean annual temperature prevails at the 

 equator, or in the fecond degree of nortii latitude. As we 

 recede from the equator, the mean temperature gradually 

 decreafes, and it is moft diminifhcd at the poles. This di- 

 minution takes place in fucli a manner, that the mean annual 

 temperatures of all the latitudes are arithmetical means be- 

 tween the mean annual temperatures of the equator and of 

 the pole. Tlie ratio between the decreafe of temperature, 

 and the diftance from the equator, was firft afcertained by- 

 Mr. Tobias Mayer of Gottingen (Oper. Ined. vol. i.); and 

 by means of an equation deduced from it, and rendered more 

 clear, accurate, and general, Mr. Kirwan has calculated the 

 mean annual temperature of every degree of latitude between 

 the equator and the pole. He fuppofes the mean annual 

 heat to be the greateft under the equator, and leaft under 

 the poles ; that at the equator he calls m, and that at the 

 north pole m — n, and putting ? for any other latitude, the 

 temperature of that latitude will be m — iiCm.(p\ Hence, as the 

 mean annual temperature of lat. 40°, determined by the 

 beft obfervations, is 62", and the temperature of lat. 50° is 

 found to.be 52.9° ; thus the value of ot and n being known, 

 the mean annu.al temperatures of the equator, and of thff 

 poles, may be determined ; for the fquare of the fine of 40"* 

 is 0.41, and the fquare of the fine of 50' is 0.5S : then, 

 m — 0.41 n = 62 

 and m — 0.58/1 = 52.9 ; 

 confequently 62 -j- 0.41 n = 52.9 -|-c.58». 

 Whence the value of n is found to be ^^ nearly, and m in the 

 fii-ft equation is 84; and therefore the mean temperature of 

 lik the 



