82 Dr. J. Croll on the Physical Theory 



much that Mr. Wallace has advanced in explanation of geolo- 

 gical climate I fully agree ; but I am, nevertheless, wholly 

 unable to perceive that any of his arguments or considera- 

 tions do in reality materially affect the theory advocated in 

 c Climate and Time/ This I hope presently to show. 



Before proceeding, however, to examine in detail Mr. Wal- 

 lace's modifications of the theory, it may be as well to consider 

 one or two minor points on which I differ from him, as this 

 will save the necessity of referring to them when we come to 

 discuss his main argument. 



Effect of Winter Solstice in Aphelion .— At page 126 (^ Island 

 Life') he says: — "We may therefore say generally, that during 

 our northern winter, at the time of the glacial epoch, the 

 northern hemisphere was receiving so much less heat from the 

 sun as to lower its surface-temperature on an average about 

 35° F., while during the height of summer of the same period 

 it would be receiving so much more heat as would suffice to 

 raise its mean temperature about 60° F. above what it is now." 

 In a footnote he adds that "the reason of the increase of 

 summer heat being 60° while the decrease of winter cold is 

 only 35°, is because our summer is now below and our winter 

 above the average." 



There is surely a confusion of ideas here. It is of course 

 true that, as our summer at present occurs in aphelion and our 

 winter in perihelion, the temperature of the former is below 

 and that of the latter above the average ; but this can afford 

 no grounds for the result Mr. Wallace attributes to it unless it 

 be assumed (for which there are no astronomical grounds) 

 that our summer is 25° further below the average than our 

 winter is above it. 



On the Storage of Cold. — In a section on the Effects of Snow 

 on Climate, Mr. Wallace points out the different effects pro- 

 duced by water falling as a liquid in the form of rain and as a 

 solid in the form of snow. The rain, however much of it may 

 fall, runs off rapidly, he states, without producing any perma- 

 nent effect on temperature. But if snow falls, it lies where it 

 fell, and becomes compacted into a mass which keeps the earth 

 below and the air above, at or near the freezing-point. When 

 the snow becomes perpetual, as on the summits of high moun- 

 tains, permanent cold is the result ; and however strong the 

 sun's rays may be, the temperature of both the air and the 

 earth cannot possibly rise much above the freezing-point. 

 "This," he says, "is illustrated by the often-quoted fact that 

 at 80° N. lat. Captain Scoresby had the pitch melted on the 

 one side of his ship by the heat of the sun, while water was 

 freezing on the other side owing to the coldness of the air." 



