1851.] HOPKINS ON CHANGES OF CLIMATE. 65 



this latter temperature at a certain elevation above the earth's sur- 

 face. A line on the side of a mountain, or an imaginary line in space, 

 along which the mean annual temperature is 32° F., I define as the 

 line of 32° F. Its height can be approximately calculated for any 

 place at which the mean annual temperature is known. In sufficiently 

 low latitudes it will be at a considerable height above the earth's sur- 

 face, but will descend to the surface along the mean annual isother- 

 mal of 32° F. as we proceed into higher latitudes. The relative 

 heights of this line and of the snow-line at the present time depend 

 on circumstances. It is essential to ascertain these circumstances and 

 their influence, that we may be the better able to estimate the height 

 of the snow-line in the hypothetical cases which I purpose to consi- 

 der. We shall then be able, as intimated in the preceding paragraph, 

 to estimate the height above the level of the sea to which the ancient 

 glaciers may have descended. The second Section of this Part will 

 be occupied with the consideration of these points. In the third and 

 final Section I shall oifer some observations on the relative claims of 

 the different hypotheses of the first two Sections to form the foun- 

 dation of geological theories. 



§ I. On the Positions of the Isothermal Lines in the above- 

 mentioned hypothetical cases. 



1 1 . Taking the first case, that of the absence of the Gulf-stream, 

 let us trace the probable course of the January isothermal of 32° F. 

 (see Map). Proceeding from east to west, we observe that it attains 

 its most southerly point on the high table-lands of south-eastern Asia, 

 proceeding thence with a little inclination towards the north until it 

 has passed the Black Sea and arrived at the longitude of 20° E. As 

 far as this point we may assume its position to be unaffected by the 

 influence of the Atlantic waters warmed by the Gulf-stream. This 

 influence, however, begins to show itself immediately to the west of 

 the point above-mentioned, in the irregular and extensive deflection 

 to the northward, which there begins to characterize this isother- 

 mal. This deflection is not entirely attributable to the Gulf-stream, 

 for the Atlantic Ocean, independently of any warming currents, 

 would undoubtedly produce some effect in lessening the winter cold 

 of western Europe, and therefore produce northern inflections of the 

 winter isothermals. Another reason for the more northern position of 

 this line in western Europe than in south-eastern Asia is the absence 

 of that high table-land in the former region which characterizes the 

 latter. To represent these influences, I have drawn the isothermal 

 for our supposed case so as to continue to the coast the general 

 northward direction which the actual line acquires about the 20th 

 degree of longitude (see Map). This causes it to meet the Atlantic 

 on the extreme western coast of Brittany. If we should draw the 

 isothermal directly west from the meridian of 20° of long., it would 

 cause the isothermal of about 24° F. to pass through that point of 

 the coast through which the line of 32° F. passes as I have drawn it, 

 so that 8° is thus allowed for the influence of the Atlantic Ocean, 



VOL. VIII. — PART I. F 



