SECOND REPORT — 1832 . 
222 
in the strata above that of invariability. The heat of the solar 
rays is constantly acting on it during the day, and with an inten¬ 
sity depending upon the absorbent power of the surface, and 
the latitude. M. Pouillet, from some ingenious experiments, 
concludes that the solar rays which reach the surface of our 
globe in the course of a year, have sufficient intensity to melt 
a complete stratum of ice over its whole extent of 14 metres in 
thickness *. The amount of solar radiation in various parts of 
the globe, presents almost an open field for investigation. Mr. 
Daniell, in his Meteorological Essays , started the apparently 
paradoxical opinion that the force of solar radiation increases 
from the equator to the poles ; and though his reasonings have 
been opposed by an eminent French philosopher f, and by me¬ 
teorologists at home J, w 7 e think he has at least had the merit 
of pointing out the fact, that the force of radiation is much less 
in the equatorial and much greater in the polar regions than 
might have been anticipated. Dr. Richardson has made some 
very interesting though not quite decisive experiments on this 
subject in the late Northern expedition§. I am indebted to 
Mr. (now Sir John) Herschel, for the remark, that observations 
on solar radiation seem generally to have been made upon an 
erroneous principle ; the true indication of the force of the so¬ 
lar rays, not being the statical effect upon the thermometer, but 
their momentary intensity measured by the velocity with which 
they communicate heat to an absorbent body. We may con¬ 
fidently point out the subject of radiation as one which will 
reward the researches of the Meteorologist. 
When the immediate calorific cause no longer acts, the sur¬ 
face of the globe of course begins to radiate the superfluous heat 
which it had received, and exactly in proportion to the facility 
with which it received it, the absorbent and emissive powers of 
surfaces being equal. Hence a nocturnal radiation of heat takes 
place from the soil, occasioning that cold wffiich, according to 
the laws known to regulate this process, is materially affected 
by the purity of the sky; for it is perfectly certain that clouds 
or any interposed body, or even the finest films, have a sensible 
influence in intercepting the rays of invisible heat. To the 
coolness thus produced in bodies exposed at night to a clear 
sky the phenomena of dew have been accurately attributed by 
Dr. Wells in his Essay on that subject,—a work undoubtedly 
* Elemens de Physique , ii. 704. f Annales de Chimie, Aug. 1824. 
+ Edinburgh Philosophical Journal, xiv. 
§ Franklin’s Second Journey, 4to edit. 
