TRANSACTIONS OF SECTION C. 661 



different method. The comparative amounts of solar heat for the various 

 latitudes are calculated (a) for the ' glacial ' winter of 199 days and (b) for the 

 coldest 190 days of our present winter. The result may he expressed thus : In 

 the 'glacial' winter latitudes 40°, 50°, 60°, 70°, and 80° receive about as much heat 

 in their 199 coldest days as 44°-5, .54°, G4°, 74°, and 85° receive in the 199 coldest 

 days at present. Hence, so far as solar heat is concerned, the utmost effect of the 

 eccentricity would he to shift the winter isothermals by 4° to the south, the 

 summer isothermals being shifted by a far greater amount to the north. Hence 

 the astronomical theory cannot account for a shift of the isothermals by more than 

 4° S. in winter and an average of more than 10° N. in summer — a result ludicrously 

 inadequate to produce an ice age in Great Britain or Ireland, or in the United 

 States, unless, indeed, as a result of the ' glacial ' winter, latitude 50° receives vastly 

 less heat from the equatorial regions by air and ocean currents than latitude 54° at 

 present receives in an equal time ; and the general principle that the passage of 

 heat to equalise temperature is greater the greater the ditterence of temperature 

 between the hot and the cold body would, if applicable here, show that 50° in the 

 glacial period should receive even more heat than 50° now, so that we should not 

 expect as great a shift as 4°, even without taking account of the increased summer 

 temperature. 



When we examine the genial age by this method the result is still moie 

 remarkable. The calculations in the paper show that the summer and winter 

 isothermals would be shifted hy about ii|° only of latitude north and south 

 respectively. It seems entirely out of the question that any change, direct or 

 indirect, depending on so slight a cause could enable walnut trees to flourish in 

 Greenland. 



After a further examination of the astronomical theory as put forward by 

 Croll, the paper deals with the relation of existing ice-fields to isothermals, to 

 isobars, and to contour lines ; and the belief of the author is that, while no 

 climatic changes due to causes known to be active at present could account for 

 the genial age, a local glacial period might easily follow from changed barometric 

 conditions, combined with a gradual elevation of land in a northern latitude. 

 Hence it would appear that either genial periods and glacial periods are due to a 

 shift of the pole, or else glacial periods are due primarily to elevation of the land, 

 while the genial age was due to greater solar activity and greater terrestrial heat 

 in the earlier geological ages. 



7. On the Mechanics of an Ice-sheet. By Rev. J. F. Blakk, M.A., F.G.S. 



The author attempted to explain how an ice-sheet can carry boulders up a 

 slope, and leave them at a height of 1,000 feet or more above sea-level. The sides 

 of the channel are, in the first instance, supposed to be parallel, so that the mass of 

 ice may be represented in a diagram by its longitudinal section. Taking for 

 simplicity the shape of the surface moved over to be represented by two 

 straight lines, one corresponding to the slope down from the mountains, the other 

 the slope up from the sea bottom to the final destination of the boulders, and taking 

 the surface of the ice as flat, the ice-sheet is represented by a triangle. This is 

 supposed to settle down in such a way that though the level of the end is higher, 

 the ceutre of gravity of the whole is lower. This fall of the centre of gravity is 

 the ettective cause of the motion of the ice-sheet, the resistance to be overcome 

 being that of the ice to change its shape. If the ice-sheet be supposed divided 

 into strips parallel to the slope from the mountains, these will be like a series 

 of overlapping glaciers, and under the influence of the pressure will swell out 

 at the bottom, and thus push the further end of the whole mass a little way up 

 the counter-slope. Continual additions of snow at the end where the ice-sheet 

 commences, or elsewhere on its surface, will be cumulative in their effects, and thus 

 the further end of the ice-sheet will ultimately ascend as required. Again divide 

 the triangle into strips by lines parallel to the counter-slope. The lower of these 

 strips will be pressed together, and any point on the base will be carried on in the 



