Physical Theory of Secular Changes of Climate. 489 
periods is only 21,000 years; for the mean rate of motion o 
the perihelion during the Glacial Epoch was considerably less 
than has been assumed. It will be seen from the table of the 
‘Longitude of the Perihelion, given in ‘Climate and Time,’ 
p. 320, that it has taken the perihelion 231,000 years to make 
one complete revolution. - 
If, therefore we assume, what of course is not certain, that 
