in Geological Climatology. 243 
the quantity received from all other sources is quite insignifi- 
cant in comparison." 
Surely Professor Newcomb must hare forgotten all about 
the researches of Pouillet and Herschel into what has been 
termed the " Temperature of Space/' or he could not have 
affirmed 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, caving some reason 
tor tne assertion. 
I 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 
from me in regard to the heat conveyed by aerial currents. 
On the Heat conveyed by Aerial Currents. — I stated that the 
quantity of heat conveyed from equatorial to high temperate 
and polar regions is trifling in comparison with that conveyed 
by ocean-currents; for the heated air rising off the hot 
ground of the equator, after ascending a few miles becomes 
exposed to the intense cold of the upper regions, and having 
to travel polewards for thousands of miles in those regions, it 
loses nearly all the heat which it brought from the equator 
before it can possibly reach high latitudes. To this Professor 
Kewcomb objects as follows: — " He (Mr. Croll) speaks of the 
hot air rising from the earth and becoming exposed to the 
intense cold of the upper regions of the atmosphere. But 
what can this cold be but the coldness of the very air itself 
which has been rising up ? If the warm air rises up into the 
cold air, and becomes cooled by contact with the latter, the 
latter must become warm by the very heat which the former 
loses ; and if there is a continuous rising current the whole 
region must take the natural temperature of the rising air. 
This temperature 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. Hav- 
ing thus got his rising current constantly cooled off by contact 
icith the cold air of the upper regions, it has to pass on its 
journey towards the poles," etc. (p. 267)*. 
Here 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 ascend- 
ing air communicates its heat by contact, is assumed to have 
* Tlie italics are mine. 
T2 
