with Geographic Botany, — Temperature* 173 



that the greatest cold is not to be found in the vicinity of the 

 poles. It is supposed that a centre of greatest cold may exist 

 in each of the continents of Asia and America^ a theory founded 

 on the well-known cooling effects of all large masses of land 

 in high latitudes. 



The information conveyed in a knowledge of the mean 

 temperature* is very slender. Standing alone, it gives no 

 idea of the distribution of heat throughout the year. Indeed 

 it must be confessed, that a complete acquaintance with the 

 vicissitudes of any one spot, during twelve months, embraces 

 a considerable number of details. When acquainted with the 

 mean heat, the chief point gained is an idea of the situation 

 of any particular place on the globe with respect to latitude ; 

 our previous knowledge fills up an outline of the climate. 

 By no limited number of facts can much desirable informa- 

 tion be conveyed. Perhaps the most complete will be ob- 

 tained in an expression of the annual range of temperature. 

 Even this will often be found not very satisfactory. 



The range of temperature throughout the year, like the 

 annual mean, bears a certain relation to the latitude. In low 

 latitudes the range is comparatively small ; in high it is often 

 very great. Near the equator, and within the tropics, the 

 range of heat is very trifling ; here the changes of a day are 

 almost those of a year. For some hours after the disappear- 

 ance of the sun's rays, the temperature falls but little ; during 

 the night gradual depression goes on, and a little before sun- 

 rise it has reached its maximum. At this time the thermo- 

 meter will stand from 15° to 30° lower than in the hottest 

 period of the day. This depression, though comparatively 

 small for its effects, has a marked influence on animated na- 

 ture; not only do the inhabitants of these warm climates 

 complain of the coldness of the atmosphere with the thermo- 

 meter at QS°, but domestic animals and the birds of the forest 

 are evidently much disturbed by it. The effects are but short, 

 the sun rises rapidly above the horizon, and the period of 

 excitement and of powerful heat again returns. It is most 

 probably owing to the exhausting power of the sun's heat and 



* Some rapid methods have been recommended to obtain the mean tem- 

 perature, where the more lengthened processes were impracticable. The 

 mean of the day may be found by meaning three observations made, just 

 before sun-rise, at 2 p.m., and at sun-set. Annual means for the latitude 

 correspond with the temperatures of considerable depths, as at the bottom 

 of the shafts of mines, and in wells. In the tropics, Boussingault advises 

 the sinking of a thermometer a foot below the surface of the soil, where it 

 is constantly shaded by a roofing. He mentions particularly the belt be- 

 tween 11° N. lat. and 5° S. lat. as suitable. In our parallel, the mean of 

 the month of October is said to be very near the annual mean. 



