LOCAL CLIMATOLOGY. IQ 



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when the sun, with the '■'■balance of tvinds" passes from one hemisphere 

 to the other. Coming north in the spring, the sun brings, or rather 

 drives before it, the warmth of his presence ; the cold winds are pushed 

 and crowded back on the poles and in the polar regions, until, of necessity, 

 a reaction takes place. Then, too, the winds that reach us at that time 

 from the pole, start in the midst of the very depths of polar winter, night 

 and cold, when the air at the pole has reached its lowest temperature. 

 It passes on its way, as soon as it reaches the sunlight at all, say in lati- 

 tude 75° or 80°, over accumulated masses of ice and snow, and frozen 

 ground; all of which, so soon as the rays of the sun become sufficient to 

 cause them to begin to thaw at all, need and will take up an immense 

 amount of heat as they pass from the solid state of ice into fluid as water. 

 Hence these winds have but little chance of getting warmed on their way 

 to us. They can scarcely grow warm at all mitil they get this side of 

 snow and frost. Then, of course, they increase in temperature quite 

 rapidly, the earth over which they pass being warmer in the day time 

 than they are ; so that after they shall have reached latitudes some 

 few degrees farther south than ours, their chilling effects can (as I should 

 presume) hardly be felt at all. Nor, if my theory is correct, can any- 

 thing of this kind be observable in Europe or Asia ; not in our latitude 

 south of the great continental mountain range, because all lands south of 

 it are sheltered by them from the winds — not north of it to any con- 

 siderable extent — because the situation of the mountains arrests the 

 northward wave, to which this is a mere reaction. And for the same 

 reason there can be no such phenomena on our Pacific coast. 



And so with the autumn or Indian summer. The sun passing south- 

 ward into the southern hemisphere, draws after it a curtain of darkness 

 and cold as it passes along ; but the earth in the southern part of the 

 temperate zone retains its warmth ; the polar current having now more 

 space by the extension of its area from north to south, becomes thinner 

 and remains the upper current longer on its way to the equator than it 

 otherwise would, thus allowing the return current to pass under it as the 

 surface current, spreading the warmth of more southern latitudes over a 

 belt extending across the Atlantic ocean, and as far west as the Rocky 

 mountains, and from north to south some ten or fifteen degrees of latitude. 

 But for the reasons already given, such a phenomenon could hardly occur 

 in the old world any more than that of the summer frosts ; and it is 

 doubtful, also, whether either of them can occur to any observable extent 

 anywhere in the southern hemisphere. The efiect of this wave and its 



