1851.] HOPKINS ON CHANGES OF CLIMATE. 67 



tical line as arising from a similar cause on tlie western coast of 

 Europe. 



Supposing the isothermal we have been considering to be correctly 

 drawn for our hypothetical case, there can be no doubt, I conceive, of 

 the approximate accuracy of the neighbouring January isothermals 

 as I have drawn them. In southern iVsia they are compressed near 

 to each other by the region of maximum cold which lies in the north- 

 eastern portion of Asia. As they approach the coast of western 

 Europe they will necessarily become more dilated, as I have repre- 

 sented them. 



12. Let us now examine the probable position of the isothermals 

 for July in the hypothetical case of the non-existence of the Gulf- 

 stream. These lines, it will be observed, as they now exist, have an 

 extraordinary inflection to the north in north-eastern Asia. As we 

 proceed westward from that region, they take a direction consider- 

 ably south of west, until they come under the influence of the ano- 

 malous temperatures of western Europe. This influence, however, 

 does not sensibly extend so far southward in summer as in winter, on 

 account of the higher temperature of the northern Atlantic in sum- 

 mer. The July isothermal of 63°* 5 F., which passes immediately 

 south of London, seems not to feel it in any sensible degree. This 

 isothermal, therefore, and all those to the south of it may be consi- 

 dered to have the same positions in our hypothetical case as in the 

 existing one. Those immediately on the north of the isothermal of 

 6J°*5 must necessarily be approximately parallel to it. We observe 

 also, as these lines approach the coast of America, they suifer an 

 anomalous deflection to the south, due, I imagine, to the polar cur- 

 rent setting southwards along that coast from Davis's Straits, the 

 warm season being that in which this cold current would be most 

 felt. I have drawn the isothermals for our supposed case, as inde- 

 pendent of these anomalous deviations, and such as their actual po- 

 sitions on the east and west of the region of these irregular influences 

 obviously indicate. It would hence appear, that the Gulf-stream has 

 no sensible influence on the July temperature of London, or of places 

 in western Europe further to the south. 



13. We are now prepared to estimate the effect produced by the 

 Gulf-stream on the mean annual temperature of any assigned place. 

 The following are the approximate numerical values of the tempera- 

 tures for January and July, and the mean annual temperature, con- 

 sidered as the mean of those two temperatures, for the Alps, Snow- 

 don, the northern extremity of Scotland, and centre of Iceland ; both 

 for the present time, and for our hypothetical case, in which, it will 

 be recollected, the configuration of land and sea is supposed to be 

 the same as at present, but the Gulf-stream not to exist. The tem- 

 peratures are all determined by Dove's map. 



r2 



