66 • HISTORY of the SOClETr. 



which contain many excellent examples of generalization, in a 

 branch of natural hiftory where it is more eafy to accumulate 

 fads, and more difficult to afcertain principles, than in any 

 other *. 



After 



• It may be proper to mention here fome ufeful obfervatlons in meteorology 

 which Dr HuiTON made, but of which he has given no account in any of his pub- 

 lications. 



He was, I believe, the firft who thought of afcertaining the medium tempera- 

 ture of any climate by the temperature of the fprings. With this view he made 

 3. great number of obfervations in different parts of Great Britain, and found, by 

 a Angular enough coincidence between two arbitrary meafures, quite independent 

 of one another, that the temperature of fprings, along the eaft coafl of this ifland, 

 varies nearly at the rate of a degree of Fahrenheit's thermometer for a degree 

 of latitude. This rate of change, though it cannot be general over the whole 

 earth, is probably not far from the truth for all the northern part of the temperate 

 zone. 



For cllimnting the efFeft which height above the level of the fea has in diml- 

 nifhing the temperature, he alfo made a feries of obfervatlons at a very early pe- 

 riod. By thefe obfervations he found that the diiference between the ftate of the 

 thermometer in tvi'o places of a given difference of level, and not very diftant, in 

 a horizontal direiSion, is a conftant quantity, or one which remains at all feafons 

 nearly the fame, and is about i° for 230 feet of perpendicular height. 



I MUST, however, obferve, that on verifying thefe obfervations, I have found 

 the rate of the decreafe of temperature a little flower than this, and very nearly a 

 degree for 250 feet. This feems to hold for a conliderable height above the 

 earth's furface, and will be found to come pretty near the truth, to the height 

 of five or fix thoufand feet. It is not however probable that the diminution of 

 the temperature is exaftly proportional to the inci-eafe of elevation ; and it would 

 feem that at heights greater than the preceding, the deviation becomes fenfible ; 

 the differences of heat varying in a lefs ratio than the differences of elevation. 



In explaining this diminution of temperature as we afcend in the atmo- 

 fphere, Dr Hutton was much more fortunate than any other of the philofophers 

 who have confidered the fame fubjeft. It is well known that the condenfation of 

 ■air converts part of the latent into fenfible heat, and that the rarefaftion of air con- 

 verts part of the fenfible into latent heat. This is evident from the experiment of 

 the air-gun, and from many others. If, therefore, we fuppofe a given quantity of 

 air to be fuddenly tranfported from the furface to r.ny height above it, the air will 

 expaiKl on account of the diminution of preffure, and a part of its heat becoming 



latent, 



