182 Mr. J. Croll on Ocean- currents in relation to the 



ocean, yet brings into operation physical agencies which can 

 and do affect such currents to an enormous extent. 



With the eccentricity at its superior limit and the winter 

 occurring in the aphelion, the earth would be 8,641,876 miles 

 further from the sun during that season than at present. The re- 

 duction in the amount of heat received from the sun owing to his 

 increased distance would, upon the principle we have already 

 stated, lower the midwinter temperature, other things being 

 equal, about 46°. In temperate regions the greater portion of 

 the moisture of the air is at present precipitated in the form of 

 rain, and the very small portion which falls as snow disappears 

 in the course of a few weeks at most. But in the circumstances 

 under consideration, the mean winter temperature would be low- 

 ered so much below the freezing-point that what now falls as rain 

 during that season would then fall as snow. But this is not all; 

 the winters would then not only be colder than now, but they 

 would also be much longer. At present the winters are nearly 

 eight days shorter than the summers ; but with the eccentricity 

 at its superior limit and the winter solstice in aphelion, the 

 length of the winters would exceed that of the summers by no 

 fewer than thirty-six days. The lowering of the temperature 

 and the lengthening of the winter would both tend to the 

 same effect, viz. to increase the amount of snow accumulated 

 during the winter; for, other things being equal, the larger the 

 snow-accumulating period the greater the accumulation. I may 

 remark, however, that the absolute quantity of heat received du- 

 ring winter is not affected by the decrease in the sun's heat* ; for 

 the additional length of the season compensates for the decrease 

 in the sun's heat. As regards the absolute amount of heat re- 

 ceived, increase of the sun's distance and lengthening of the 

 winter are compensatory, but not so in regard to the amount of 

 snow accumulated. 



The consequence of this state of things would be that, at the 

 commencement of the short summer, the ground would be covered 

 with the winter's accumulation of snow. 



Again, the presence of so much snow would lower the summer 

 temperature and prevent to a great extent the meltingof the snow. 



There are three separate ways whereby accumulated masses of 

 snow and ice tend to lower the summer temperature, viz. : — 



■First i The snow and ice lower the temperature by means of 

 direct radiation. No matter what the intensity of the sun's 

 rays may be, the temperature of the snow and ice can never rise 

 above 32°. Hence the presence of the snow and ice tends by 



* When the eccentricity is at its superior limit, the absolute quantity of 

 heat received by the earth during the year is about one three- hundredth 

 part greater than at present. But this does not affect the question at issue. 



