390] 
we should feel the same degree of 
cold in all regions, after we have 
risen to acertain height, say 24,000 
fet ; considering that the variations 
with respect to heat, on the.earth, 
not onlv in the different climates, 
but in the same country, at differe 
ent seasons of the year, are so per. 
ceptib.e. This variety, which takes 
place at the surface of the globe, 
is undoubtedly occasioned by the 
sun. It appears, at fiist sight, 
tha: his influence must be the same 
above ad b low, especially when 
we reflect, that a height of 24,000 
eet, ora mile, though very great 
with respect to us, and even far 
beyond the height of the loftiest 
mountains, isa mere nothing, com- 
pared to the distance of the sun, 
which is about thirty milltens cof 
miles*, This.is therefore, a very 
important difficulty, which we 
must endeavour to solve. For this 
purpose, 1 begin with remarking, 
thit the rays of the sun do not 
communicate heat to any hodies, 
but such as do not grant them a 
free passege. You know that 
bodies, through. which we can dis- 
cern objects, ave denominated trens- 
parent, plincid, and diaphonous. 
These bedies are glass, crystal, 
diamond, water, and several other 
liquids, though some are more or 
less transparent than ovhers, Cne 
of these transparent bodies being 
exposed to the sun, is not heat da 
tosuch a degree as a bedy not trans- 
parent, as wood, iron, a Bodies 
not transparent are ominared 
opzque. <A burning ae fet ex 
ample, by trensmitring tbe ravs of 
the sun, sets on fire opaque bodies, 
while the glass itself ts not sensibly 
heated. Water exposed to the sun 
ANNUAL REGISTER, 
1796. 
becomes somewhat warm, only be- 
cuu-e it ts not perfectly transparent; 
when we see it cons.derably heated 
by the sun at the brink of ri- 
vers, it is because the bottom, be~ 
ing an opaque bedy, is heated by 
the rays which the water transmits, 
Now, every heated body commu. 
picates that h at to all adjoining 
bodies ; the water according!y de- 
rives heat from the botrom. [i the 
water be very deep, so that the rays 
cannot penetrate to the bottom, it 
has no perceptible heat, though the 
sun bears upon it. 
As airisa very transparent body, 
to a much higher degree than glass 
or, water, it follows, that :t cannct 
be heated by the san, because the 
rays are freely transmitied through 
it. The heat which we frequently 
feelin the air, 1s communicaicd to 
it by opaque bodies, which che rays 
of the sun have -heated ; and were 
it possibie to annihilate all these 
bodies, the air wou'd scarcely un- 
Gergo any change in its temperature 
by the rays of the sen: exposed to 
it or not, tt would be equally cold. 
But the atmosphere j is not perfecily 
transparent : it is even some:imes 
so loaded with vapours, that 1: loses 
almost entirely iis transparency, 
and presents only a thick fog. 
When the air isin this state, the 
rays of the sun have a more power. 
ful influence upon it, and heat it 
immediately. 
Burt these vapours rise to no 
great height; at the height of 
£4,000 fret, nail beyond, the air 
is sosubtile and so pure, that it is 
perfeétiy transparent ; and for this 
reason the rays of the sun cannot 
immediately produce any effect 
upon it. This air is likewise too 
3 Mr. Evler always means German miles, of 4000 fathoms each, or somewhat 
under 43 miles English —E.E. 
remote 
