J. Croll— Geological Climatology. 251 
termed the ‘Temperature of Space, or he could not have 
ed so positively that ‘‘ practically there is but one source 
from which the earth receives heat, and that all other sources are 
quite insignificant” without,.at least, giving some reason for 
the assertion. 
am pleased to find that he agrees, in the main, with what 
has been advanced in ‘ Climate and Time,’ in reference to the 
heating power of ocean currents, and also as to their existence 
being due to the impulse of the winds. But he differs widely 
mperature is, indeed, much below that which maintains 
at the surface for the simple reason that air becomes cold by 
expansion according to a definite and well-known law. Havy- 
ing thus got his rising current constantly cooled off by contact 
with the cold air of the upper regions, it has to pass on its 
journey towards the poles,” etc., p. 267.* 
ere the cooling of the ascending air is attributed to two 
Causes (1) the heat lost by expansion as the air rises ; (2) the 
heat lost by contact with the colder air through which the 
ascending air passes and with which it mixes in the upper 
regions. But the two may be resolved into one, viz: the heat 
lost by expansion ; for the cold air to which the ascending air 
communicates its heat by contact, is assumed to have origi- 
nally derived its cold; in like manner from expansion. This 
18 evident, for although he recognizes the effect of radiation 
into space, he assumes that this loss is compensated by coun- 
ter radiation, The upper regions are, he says, exposed to the 
* The italics are mine. 
& 
