476 METEOROLOGY. TEMPERATURE 



of the intertropical plants are in this predicament. There are others 

 which, imperatively requiring a high temperature for their growth 

 and perfection, nevertheless suspend their powers during the winter, 

 and bear without detriment degrees of cold of great intensity : 

 among the number may be cited the larch-pine, which abounds in 

 Siberia, and stands the utmost rigors of its climate, where the 

 thermometer at mid-winter frequently falls to 30° and even 40^ be- 

 low zero, F. 



The meteorological habitudes or dispositions of plants being ex- 

 tremely various, it follows, that the geographical distribution of 

 plants is a consequence of the distribution of heat over the surface 

 of the globe — of climate. 



The earth we inhabit appears to have a hsat proper to itself ; it is 

 a heated body in progress of cooling. It is found, in fact, that as 

 the centre of the earth is approached, as mines penetrate more 

 deeply below its surface, the temperature increases. Below a very 

 limited distance from the surface, the temperature ceases to be 

 affected by variations in the temperature of the general atmosphere ; 

 from the pi)int of invariable temperature the subterranean heat in- 

 creases uniformly at the 'rate of 1^ cent. (1.8^ Fahr.) for every 101 

 feet of descent. 



The depth at which the point or stratum of invariable temp raturn 

 is met with, varies in ditferont places, and is mainly affected by the 

 extent of the thcrmomctrical variations in the superincumbent air in 

 the course of the year. In the higher latitudes, consequently, the 

 depth is very considerable ; at Paris, for example, >[. Arago has 

 found that a thermometer, buried at 20] feet under the surface, d')es 

 not reman absolutely stationary. In climates of greater constancy, 

 as may be conceived, tlie layer of invariable temperature will be 

 found much nearer the surface ; were the temperature of the air in- 

 variable, the layer of invariable temperature would ncccs.«arily be- 

 found at the surface of the ground. In countries under and clo.sc to 

 the equator, this, in fact, is found to be the case. From a series of 

 observations which 'I mal.; in So ith AniM-ica, b;^tweeii the 21 paral- 

 lel of southern and the 11th of northern latitude, I found that, near 

 the line, the layer of invariable temperature is found nearly at the 

 surface ; the thermometv^r, ])Iaced in a hole al>out one foot deep, 

 under the shade of an Indian cabin, or a shol, does not vary by more 

 than from one tenth to two tenths (»f a degree Cent. 



It was probably under the influence of the internal or proper heat 

 of the globe, according to ^[. do Humboldt, that the same species 

 of animals which are now confmed to the torrid zone, inhabited, in 

 former and remote ages, the northern hemisphere, covered as it then 

 was by arborescent ferns and stately palms. It is easy to imagine 

 how, as the surface of the earth cookvl, the distribution of climatt'S 

 became almost exclusively dependent on the action of the .«;olar rays, 

 and how also those tribes of plants and of animals, the organization 

 of which required a higher temperature and more cciuable climate 

 gradually died out and clisappoared.* 



♦ IIumboldt'9 Central Asia, v. til., p. 98. 



