Mr. J. Croll on the Physical Cause of Ocean-currents. 185 



strength of the currents, would not, however, greatly affect the 

 mode of distribution of the winds, and, as a consequence, the 

 mode of distribution of the currents. But although a simple 

 change in the difference of temperature between the equator and 

 the poles would not produce a different distribution of^ aerial, 

 and consequently of ocean-currents, nevertheless a difference in 

 the difference of temperature between the equator and the two 

 poles would do so j that is to say, any cause that should increase 

 the difference of temperature between the equator and the pole 

 on the one hemisphere, and decrease that difference on the 

 other, would effect a change in the distribution of the aerial 

 currents, which change would in turn produce a corresponding 

 change in the distribution of ocean- currents. 



It has been shown* that an increase in the eccentricity of the 

 earth's orbit tends to lower the temperature of the one hemi- 

 sphere and to raise the temperature of the other. It is true 

 that an increase of eccentricity does not afford more heat to the 

 one hemisphere than to the other ; nevertheless it brings about, 

 as I have already shown, a condition of things which tends to 

 lower the temperature of the one hemisphere and to raise the 

 temperature of the other. Let us imagine the eccentricity to 

 be at its superior limit, '07775, and the winter solstice in the 

 aphelion. The midwinter temperature, owing to the increased 

 distance of the sun, would be lowered enormously; and the effect 

 of this would be to cause all the moisture which now falls as rain 

 during winter in temperate regions to fall as snow. Nor is this 

 all ; the winters would not merely be colder than now, but they 

 would also be much longer. At present the summer half year 

 exceeds the winter half year by nearly 8 days ; but at the period 

 in question the winters would be longer than the summers by 

 upwards of 36 days. The heat of the sun during the short 

 summer, for reasons which have already been explained, would 

 not be sufficient to melt the snow of winter ; so that gradually, 

 year by year, the snow would contiDue to accumulate on the 

 ground. 



On the southern hemisphere the opposite condition of things 

 would obtain. Owing to the nearness of the sun during the 

 winter of that hemisphere, the moisture of the air would be pre- 

 cipitated as rain in regions where at present it falls as snow. 

 This and the shortness of the winter would tend to produce 

 a decrease in the quantity of snow. The difference of tem- 

 perature between the equatorial and the temperate and polar 

 regions would therefore be greater on the northern than on 

 the southern hemisphere ; and, as a consequence, the aerial 



* Phil. Mag. August 1864, February 186/, March 18/0, and other 

 places. 



