S. Newcomh—CroUs Climak and Time. 271 



hemisphere, considering only the heat of the sun, we shall have 

 a short and hot summer, and a long and cold winter, while in 

 the southern hemisphere the summer would occur when the 

 sun was farthest off, and the winter when he was nearest, so 

 that the rigor of both seasons would be greatly mitigated. 

 Still, as just stated, the mean temperature due to solar heat 

 would be the same in each hemisphere, the northern hemi- 

 sphere having the hotter summer as well as the colder winter. 



While admitting this equality in the total amount of heat 

 received from the sun, Mr. Croll endeavors to show that the 

 northern hemisphere would be colder and that there would be 

 an accumulation of snow and ice during the long winter which 

 would not be melted by the sun's rays during the short and 

 hot summer. His principal arguments in favor of this view 

 are given in Chapters II and IV. Beginning with the winter 

 he says that the reduction in the amount of heat received from 

 the sun during this season, owing to his greater distance, would 

 lower the mid-winter temperature to an "enormous extent." 

 Precisely how great a diminution of temperature he considers 

 enormous he does not state in this connection, but in a subse- 

 quent chapter he computes a diminution in the mid-winter 

 temperature of Great Britain sometimes amounting to more 

 than 30= from this cause. This computation we regard as en- 

 tirely untrustworthy, being founded on purely hypothetical 

 laws with purely hypothetical data, but we need not challenge 

 the result at present. The effect of this lowering of the tem- 

 perature would be, he says, a great increase in the amount of 

 snow which would fall during the winter. This conclusion we 

 cannot accept During the long cold winter the evaporation 

 must be lessened, and hence the amount of precipitation also, 

 unless warm and moist air is brought from the warmer regions 

 of the globe. In this case the latent heat set free by precipita- 

 tion, as well as the heat of the air itself, would mitigate the 

 wmter temperature. On the whole, we may consider the 

 equivalent of twenty inches of solid ice to represent a very lib- 

 eral estimate of the probable average amount of snow which 

 would accumulate over any very great extent of surface during 

 any one winter. 



Mr. OrolFs next point is that the presence of so much snow- 

 would lower the summer temperature, and prevent to a great 

 extent the melting of the snow. He gives three reasons for 

 this extraordinary proposition. In the first place the air will 

 be cooled by radiation to the snow more rapidly than it will be 

 heated by the sun. Of the fact that this cooling of the air 

 would itself be necessarily accompanied by a meltmg of the 

 snow he shows no consciousness. A simple calculation will 

 sbow that a cooling of the air by some 30° Fahrenheit would 

 melt the whole twenty inches of snow. 



