264 G. W. Bulman — Revised Theory of GJaciation. 



regions, it is still sufficiently greater than that of Greenland to make 

 it difficult to believe that a mean winter temperature of i^° would 

 reduce our country to the present state of the latter. 



We may, then, conclude that a mean winter temperature of 21°, 

 or even of 9°, does not necessarily iaiply glacial conditions Wee those 

 of Greenland at present. 



From Sir K. Ball's data again, the average summer temperature 

 during the most genial period possible, viz. when the summer is 

 199 days and the winter 160, can be calculated. The present mean 

 daily heat of summer is 1*24, and during the summer of 199 days 

 it would be 1*16, This gives a reduction of 0-08 of a heat unit, 

 and since each tenth of a unit corresponds to a fall of 30°, we have 

 i-o-x30°=:24° as the difference in average temperature between our 

 summer and that of the most genial period. If we take the July 

 isotherms as rough guides to the average summer temperature, this 

 would give us a Greenland sitmmer during the most genial period. 

 Such a summer climate can scarcely be supposed to have tempted 

 tlie plants and animals of the tropics or subtropical regions to wander 

 north to our latitudes. 



If, then, the burden of accounting for tropical, or subtropical, as 

 well as glacial periods is to be laid on the Astronomical Theory of 

 Glaciation, the revised form must be declared wanting. 



But these reductions of winter and summer temperatures depend 

 largely on the number — 300, which has been used in the calculations, 

 and its use requires some justification. If the lohole yearly heat 

 supply of a hemisphere had been reduced, and not merely the daily 

 average of lointer ; or if the earth in the absence of the sun could 

 very rapidly cool down to — 300° ; then the above calculations might 

 be taken as approximately true. But the yearly heat supply'' is not 

 appreciably altered. Moreover, the earth — or particular hemisphere 

 under consideration — has a certain temperature due to the heat of 

 the previous summer ; and this temperature must share with the lointer 

 heat supply the task of maintaining the hemisphere above — 300°. 

 Suppose this temperature resulting from the heat of the previous 

 summer to be 55°. If, in the absence of the sun, the earth could 

 sink from this to — 300° in a few days, then this factor might be 

 neglected. 



But supposing that instead of the above rapid rate of cooling it 

 required, say, six months to cool down to — 100°; then, instead of 

 — 300, we should have to use — 100 in our calculations, and this 

 would make important differences in our reasonings. I am not 

 prepared to assert that — 100 is the correct number, but it seems 

 almost certain that — 300 is too low, and that the earth would not 

 sink to so low a temperature in six months. For the rate of cooling 

 of a heated body diminishes rather rapidly with the diminution of 

 its excess of temperature. Thus the rate of cooling for an excess of 

 80° C. is little more than a tenth of what it is for 240° C Again, it 

 has been shown that the rate of cooling diminishes with the density 

 of the surrounding air up to a pressure of TtroTro-o o o" of '"i" atmosphere. 

 But the earth's heat before it can radiate into stellar space must 



