59 6 



SCIENTIFIC RECREATIONS. 



nearest approach and greatest distance are termed perihelion and aphelion. 

 In the latter case we are about 9,000,000 of miles farther from the sun than 

 when in perihelion that is, when the greatest "eccentricity" is reached. In 

 addition to this the axis of the earth is continually changing in direction by 

 reason of solar attraction at the equator. This shifting, as explained in the 

 astronomical section, is very slight every year, and in the course of 24,000 

 years the conditions of the seasons will have completely changed round and 

 back again, for the northern and southern conditions will be reversed in 

 our hemisphere. Day and night come twenty minutes earlier every year. 

 We are now nearer the sun in winter as shown in diagram (page 497) ; when 

 we change we shall be nearest the sun in summer and farthest in winter. 



Fig. 686. The Mer de Glace. 



Doctor Croll, who has done much in his most interesting paper on 

 changes of climate*, tells us how this eccentricity of the earth's orbit produced 

 indirectly the Glacial epoch. He shows how, if in a period of the greatest 

 " eccentricity " our winter came in aphelion, we should receive one-fifth less 

 heat than now, but a correspondingly greater heat in summer. But if our 

 winter under such circumstances fall (as now) in perihelion, the difference 

 between winter and summer would be practically nil, because the sun during 

 a period of the earth's great eccentricity " could not warm the hemisphere 

 whose summer happened to arrive in perihelion." No doubt the sun's rays 

 would be very powerful, but the earth being covered with ice and snow 

 could not be warmed ; fogs would accrue and hide the sun, as at present in 

 Antarctic summers, when the cold is very great. The warm ocean currents 



* " Physical Causes of Change of Climate," Phil. Mag., 1864. 



