406 



ISOTHERMAL LINES. 



CHAPTER 

 XXVIU. 



Isothermal 

 lines. 



Line of 



muximuni 



teinpeniture. 



Tlii^oretical 

 an I ascer- 

 tai:ica lines 

 of tempera- 

 ture. 



th<i.t the popularly accepted zone's comparative heat,, 

 and the supposed equatorial line of highest temperature 

 can hardly be accepted even as a rude approximation to 

 the general law by which the regions of perpetual snow are 

 fixed at the poles, and those of unendurable heat at the 

 equator. An entirely new system of physical geogi-aphy 

 is rendered necessarj', in order to make this rational sys- 

 tem of ascertained truths practically available, and this 

 has already been accomplished with great learning and 

 practical skill, by Mr. Alexander Keith Johnston of Edin- 

 burgh. As a practical synopsis, not only of this depart- 

 ment of physical science, but of all the sub-divisions of 

 geology and ethnology, Mr. Johnston's " Physical Atlas" 

 is the most valuable and comprehensive work to which 

 the scientific student can refer. 



Comparing ascertained facts relative to the meteorolo- 

 gical condition of various extensive regions, it is found 

 that the probable line of maximum temperature of the 

 atmosphere, or the atmospheric thermal equator, as it may 

 be styled, differs very considerably from the equator of 

 the earth. In the northern hemisphere it intersects 255 

 degrees of its circumference, while in the southern 

 hemisphere it onlj' intersects 105 degrees, — an amount 

 of difference which shows that there is hardly an ap- 

 proximation of the true thermal equator to the geogra- 

 phical line with which it was long held to coincide. We 

 perceive, accordingly, from this, how limited an influence 

 the mere relative position of certain portions of the earth 

 to the sun, exercise upon their actual or annual mean 

 temperature. "Were the earth, indeed, a plain solid 

 sphere, of equal level, and perfect homogeneity through- 

 out, the old theory of frigid, temperate, and torrid zones, 

 would hold true in fact ; but in so far as it in reality 

 difi'ei-s from this, does the theoretic geographical division 

 fall short of the truth in relation to the actual ascertained 

 scientific lines of temperature. The previous observa- 

 tions on the peculiar geographical and geological features 

 of the Asiatic continent will have prepared the reader 



