88 Mr Blackadder on Circumstances connected with the 
3. It has been well ascertained, that if, on a clear evening, 
for example, when bodies on the surface of the earth have be- 
come colder than the air, a cloud should pass over the zenith, 
the thermometer will indicate an increase of temperature, and, 
after the cloud has passed, it will again indicate a loss of heat. 
This increase of temperature has been accounted for in various 
ways. Some have supposed the heat to be evolved by the con- 
densation of the aqueous vapour constituting the cloud ; but it 
has not been satisfactorily explained how this heat is brought 
down to the earth, even admitting that such a quantity is evolved, 
as to render it appreciable beyond the immediate limits of the 
cloud, which, though it may be comparatively low, is still at a 
great distance from the earth. 
This increase of temperature during the transit of a cloud, has 
been accounted for on the pulsatory hypothesis, by supposing, 
that 44 clouds, like water, absorb and extinguish all the hot and 
cold pulses received by them.'” 
But were it even ascertained that 44 cold pulses,' ” or 44 fri- 
gorific rays,” were actually, 44 in some way or other, showered 
down from the upper regions of the atmosphere upon the 
earth,” the phenomenon in question could not, it is presumed, 
be satisfactorily accounted for on that principle. For, a com- 
paratively small cloud in the zenith, could not be supposed ca- 
pable of neutralising the effect of the 44 cold pulses,” showering 
in all directions from an extensive clear sky, by which the cloud 
is every where surrounded. We are informed, that the 44 cold 
pulsations” come with equal force from all quarters of the hea- 
vens, and at every angle 20° above the horizon. 
Dr Wells was of opinion, that 44 no direct experiments can be 
made, to ascertain the manner in which clouds prevent, or occa- 
sion to be small, the appearance of a cold at night upon the 
earth ;” but he concludes, 44 that they produce this effect, almost 
entirely, by radiating heat to the earth, in return for that which 
they intercept in its progress from the earth towards the heavens,” 
This is the explanation originally given by M. Prevost of Ge- 
neva, and which is grounded on his hypothesis regarding radia-* 
tion. 44 Dense clouds,” Dr Wells continues, 44 near the earth, 
must possess the same heat as the lower atmosphere, and will 
therefore send to the earth as much, or nearly as much, h£at as 
