114 The Climate of Great Britain. 



require somewhat more space than the readers of the Intellec- 

 tual Observer would willingly see allotted to a single subject. 

 I wish chiefly to consider the subject of temperature (mean 

 annual and extreme winter or summer) ; though I may, per- 

 haps, have a few words to say respecting that feature of our 

 climate, which most foreigners consider to be its chief defect — 

 the want of transparency or clearness in our skies as compared 

 with those of some other European countries. 



The mean annual temperature of a country is less im- 

 portant to the welfare of the inhabitants than the extreme 

 range of temperature exhibited in the course of the year. Of 

 two countries which have the same mean annual temperature, 

 one may have a climate most admirably adapted to the welfare 

 of its inhabitants, while the other may have a climate offering- 

 such fierce and violent extremes of heat and cold that its 

 inhabitants resemble the unfortunates described by Dante, 

 doomed 



" a soffrir tormenti caldi e geli." 



However, I shall deal first with this feature — mean annual 

 temperature — as affording a starting point from which to pro- 

 ceed to other considerations. 



If the surface of the earth were perfectly uniform, or sym- 

 metrically distributed into districts of land and water arranged 

 in zones along latitude-parallels, and if the strata of the soil 

 were throughout of like density, radiating power, and elevation, 

 the lines of equal mean temperature would be parallels of lati- 

 tude. This hypothetical condition of things is, we know, very 

 far from representing the true condition of the earth's surface. 

 Land and water are distributed in a manner which hardly pre- 

 sents the semblance of law ; elevations and depressions, not 

 merely of areas of limited extent, but of whole countries, are 

 exhibited in each hemisphere ; and endless diversities of soil, 

 contour, and distribution, disturb that mathematical unifor- 

 mity and exactness, which could alone produce the co-ordina- 

 tion of climates under latitude-parallels. 



It is to Humboldt that we owe the valuable proposition 

 that maps of the world should exhibit parallels of heat, as well 

 as latitude-parallels ; and no atlas is now considered complete 

 without maps in which isotherms, or lines of equal mean annual 

 temperature, isochimcnals or lines of equal winter heat, and iso- 

 fherals or lines of equal heat in summer, are exhibited. These 

 lines are usually presented in maps on Mercator's projection, 

 an arrangement which has some advantages, but is not, on the 

 whole, very well suited to exhibit the true conformation of the 

 isothermal lines — the study of which, it has been well remarked, 

 constitutes the basis of all climatology. 



In Figs. 1 and 2, the northern hemisphere of the earth is 



