20 Mr Forbes on Polar Temperature. 



rologists at home would do well to emulate, by ascribing the 

 monstrous deviation of twenty-five or thirty degrees to " latent 

 and material inaccuracies." 



But, upholding in the first place such a submission of theory 

 to experience, I propose, with all deference, to offer an explana- 

 tion of the supposed anomaly, that ice does not accumulate 

 under the Arctic Circle. This I shall endeavour to do by show- 

 ing, first, the difference of operation in the formation and dis- 

 solution of ice ; second, the influence of marine, — and lastly, of 

 aerial currents. 



I. That the mean temperature of a climate is the only index 

 of the preponderance of the power which forms, or that which 

 dissolves, ice, has been thought an undeniable fact, because, 

 though vegetable productions may, under other circumstances, 

 be equally matured in climates differing much in mean tempe- 

 rature, this arises from the comparative intensity of the heat 

 of a short summer, which is yet sufficiently long to bring par- 

 ticular crops to perfection. But, with regard to the summer 

 and winter effect, however distributed, upon ice, it is argued 

 that the effect must depend solely upon the mean temperature, 

 because the conversion of water into ice, and the reconversion 

 of the latter into the former, require a degree of comparative 

 cold in the one case, and of comparative heat in the other, 

 which admit of arithmetical estimation, and are precisely com- 

 mensurate. All this is undeniable ; but if we examine the mode 

 of the formation and dissolution of ice, we shall find an admir- 

 able provision of nature for preventing the former action from 

 overbalancing the latter. Suppose the sea reduced at the sur- 

 face to 28° ; if a sudden fall of temperature in the air takes 

 place, ice will be formed very rapidly, — even to several inches 

 in thickness in one night. But observe the effect of the con- 

 tinuance of cold : the thickness of the ice cannot be increased 

 on that side next the frigorific influence, namely, the atmo- 

 sphere ; the material is the sea-water, and exists only below ; 

 the influence of that intense cold, therefore, which lowers by 

 so great a number of degrees the annual mean, must be exert- 

 ed through the difficult conductor of the mass of ice already 

 formed, and the intensity of its impression is always rendering 

 more tedious the accession of cold to the sea-water, by thick- 



