194 Natural History. 



of the regions through which it passes in its de- 

 scent. 



Much light has been thrown, in the course of 

 the last century, on the varieties of tempcraturey 

 in different seasons and latitudes. On this subject 

 Dr. Halley made some instructive observations. 

 A few years afterwards, M. De Mairan, an in- 

 genious French meteorologist, by a series of ob- 

 servations and experiments, discovered that the 

 rigour of the winter's cold is tempered by the heat 

 imparted to the atmosphere by the earth itself; 

 and thus explained by what means the winter'sj 

 cold is rendered so moderate as to make the colder 

 climates inhabitable. On the ground of this dis- 

 covery he calculated, with great sagacity, the 

 maximum and minimum of heat in every latitude, 

 for the. summer and winter solstices; and though 

 his calculations are not always found to coincide 

 with facts; yet they have proved highly instructive 

 and useful to subsequent inquirers. De Mairan 

 was followed by M. Mayer, the celebrated astro- 

 nomer of Goettingen, who, in a few pages, did 

 more to solve the difficulties that occurred on 

 this subject than any of his predecessors. He 

 first pointed out to meteorologists the necessity of 

 following the method long used by astronomers; 

 namely, of first finding the mean of certain large 

 periods, as years and months, gradually correcting 

 the errors that may be discovered, and afterwards 

 finding an equation whereby to correct aberrations 

 arising from height and situation. He even pro- 

 ceeded so far as to give an equation to correct the 

 effects of height, which in many cases approximates 

 very nearly to the truth; but the equation by which, 

 knowing the men* annual temperature of two la- 

 titudes, the mean annual temperature of every 

 other latitude, and even of the pole itself, may be 



