54 * 
Notices of Books . 
[October, 
Glacial epoch may be taken at about 2300 feet.” Mr. Belt, on the 
other hand, holding that the whole globe was glaciated not by alter- 
nate hemispheres, but simultaneously, considers that the amount 
of water abstracted from the ocean, and piled up in a solid form 
upon the continents, must have been so considerable as to reduce 
the general sea-level and lay bare lands now submerged to depths 
ranging from 1000 to 2000 feet, which would afford a temporary 
asylum for the tropical flora and fauna. 
Mr. Williams seeks the cause of glacial epochs simply in 
alternating lengths of summer and winter in the two hemispheres. 
He holds that “ no shifting of the earth’s axis, no extreme con- 
vulsion, is demanded ; for if, under the present distribution of 
terrestrial climate, the southern hemisphere had as much land 
around its poles as the northern hemisphere now has, it would 
be glaciated from the South Pole down to the latitude cor- 
responding to that of London.” It appears to him that “ if we 
simply add the changes of climate which, as Lyell has shown, 
must of necessity result from variations in the distribution of 
land and water to the similar changes which must follow as 
necessary results of the known and demonstrable variations of 
the earth’s orbit, we shall have in this combination an agency of 
sufficient energy to explain all the known phenomena.” In his 
former work, “ Through Norway with a Knapsack,” Mr. Williams 
has contended that “ low temperature is only one of the faftors 
in the formation of glaciers, and that an increase of atmospheric 
humidity, and of consequent winter snow-fall, may produce the 
same effedt as lowering of temperature.” He supposes, indeed, 
that in the ice-age “ the temperature of the northern hemisphere 
was lower than at present, but not nearly so low as is commonly 
supposed ; and that this moderate depression of temperature, 
combined with a greater snowfall in winter, produced all the 
observed results. The lower temperature would favour increased 
precipitation, and the increased humidity of the air would resist 
the passage of the solar rays and greatly diminish the summer 
thawing.” 
According to Mr. Williams, then, the southern hemisphere is 
now “ enjoying” its glacial epoch, though not in a very aggra- 
vated form. But it is possible, or rather highly probable, that 
an agency which he leaves out of account may very much 
modify the character of these ice-ages. The orbit of the earth 
varies periodically, alternately approaching nearer to a circle, 
and then becoming more eccentrically elliptical. This cause, as 
Mr. Croll has shown, will sometimes greatly intensify the in- 
fluence of the shortened summer, and may thus produce epochs 
of a more intensified glaciation. 
There are other subjects, not a few, on which our author 
expresses himself in an original and interesting manner, but 
they are not adapted for discussion in the “ Quarterly Journal 01 
Science.” We will therefore conclude by thanking him for the 
