Correspondence — R. M. Deeley. 287 



CLIMATE AND TIME. 



Sir, — The discoveries of Scott and Shackleton show that ia the 

 Antarctic, as in the Arctic, the climate is ameliorating. When Scott 

 visited the Great Barrier in 1902 he found it had retreated many 

 miles since the days of Ross, and Ferrar from other evidence concluded 

 that the ice masses are gradually shrinking in size. 



This raises a very interesting point concerning the theories which 

 have been formed to account for warm and glacial periods. According 

 to Croll's theory the cold and warm periods occurred during times 

 of great eccentricity in the earth's orbit, and the cold and warm 

 periods alternated between the north and the south polar areas owing 

 to the precession of the equinoxes. In other words, the climatic 

 changes taking place must be of opposite character in the northern 

 and southern hemispheres, whereas the facts appear to be, as already 

 stated, that the climate is now ameliorating in both hemispheres. 

 Moreover, the great number of cold and warm periods Croll's theory 

 requires seems to me to be greater than is warranted by the geological 

 evidence available, 



It is admitted that the total amount of heat received by the earth 

 from the sun each year is only very slightly affected by changes in the 

 eccentricity of the earth's orbit. There is also reason to believe that 

 the obliquity of the earth's orbit has not varied appreciably. We are, 

 therefore, constrained at present to admit that, at least as regards 

 recent glacial periods, the cause of the climatic changes must lie 

 in the variable way in which the heat is received from the sun and 

 reflected or retained during the winter and summer seasons and the 

 indirect results thus brought about. 



The essence of Croll's theory is that at times of great eccentricity 

 there are periods during which the difference between the amount 

 of heat received during winter and summer is very great on one 

 hemisphere, whilst on the other hemisphere the quantities of heat 

 received during the two seasons is more equable. When the winters 

 are sufficiently cold the precipitation is almost wholly in the form 

 of snow, and if the snowfall is sufficiently great the summer sun 

 is unable to melt it ; for owing to the heat being reflected from the 

 white surface, or intercepted by fog, the part of the hemisphere 

 concerned does not profit proportionately by being nearer to the sun. 

 A great amount of heat is also required merely to change the ice 

 to water. 



Now I would suggest that during periods of great eccentricity, 

 when the winter of one hemisphere is in aphelion, the collection of ice 

 and snow is so great that the warmth of summer when the same 

 hemisphere is in perihelion is unable to melt it except in low latitudes, 

 and glacial periods are thus produced concurrently in both hemispheres 

 during times of great eccentricity. 



When, during periods of great eccentricitj', the equinoxes are in 

 aphelion and perihelion the conditions are also favourable for the 

 collection of snow and ice in both polar areas ; for during the time 

 one equinox is in perihelion the equatorial region receives a great 

 an^ount of heat from the sun, evaporation is promoted, and the 

 precipitation of snow at the poles is increased. On the other hand, 



