The Evolutio/i of Climates. — Mansoii. 159 
and exposed to an exterior source of heat and light, which 
source may be either constant or variable in its energy, but if 
the latter so slowly that it may be considered sensibly constant 
during the eras under consideration; second, the same globe 
deprived of its heat to such an extent that a crust of poor con- 
ducting material has formed, the outer surface of which is ex- 
posed only to solar heat and space, and whose climates are 
controlled thereby. The objects in view are to explain (i) 
the peculiar uniformity of climates prior to the practical ex- 
haustion of the first source, and (2) the occurrence of an age 
of general glaciation in all latitudes prior to the establishment 
of the control of the exterior source; (3) the reasons of the dif- 
ferences between heat distribution during geological and pres- 
ent climates. 
To be more explicit, we will state that the prime objects 
are to demonstrate — 
1. That in the passage of the earth from an era during 
which its climates were controlled by internal heat into an era 
during which its climates are controlled by solar heat, eras of 
uniform climates must have been passed through during w^hich 
isotherms were independent of latitude; and elevation above 
sea level was the prime cause of climatic variation. 
2. That before climates could pass under solar control an 
age must occur during which elevated continental areas must 
be glaciated; and that this phenomenon, occurring before solar 
climatic control, was also independent of latitude. 
3. That the direct cause of the Ice age was a combina- 
tion of the remarkable properties, in relation to heat and cold, 
possessed by the various forms of water. As water, by rea- 
son of its high specific heat, it longer retained the effective 
remnant of earth heat; as vapor, in the form of fogs and clouds, 
it largely prevented the loss or receipt of heat by radiation ; as 
ice, it assumed a solid form, storing matter at a low tempera- 
ture. 
We may therefore state the problem in the form of a propo- 
sition in which we accept as a fact that the earth was at one 
time a heated mass, whose resident heat expanded all water, 
and possibly other volatile matters into a vaporous and gaseous 
envelope. 
