METEOROLOGY. 
fcrcases towards the poles as the differ- 
ence of the temperature, and tlie density 
of the atmosphere increases with the lati- 
tude. The sinking of thg barometer pre- 
ceding violent tempests, and the oscilla- 
tions during their continuance, prove that 
very great rarefactions, or even destruc- 
tion of air, in some part of the atmosphere 
produce those phenomena ; the fall too that 
accompanies winds arise from the same 
Cause. Unfortunately we are but little ac- 
quainted with the operations which pro- 
duce rain, consequently we are unable to 
explain satisfactorily why the barometer 
falls immediately preceding it. 
The most inattentive observer of the 
phenomena of nature must have noticed 
that there are considerable variations in 
the temperature of the aif in any particular 
place, exclusive of the differences of seasons 
and climates, which eternal changes Cannot 
be produced by heat derived from the sun, 
as its rays concentrated have no kind of 
effect on air, those however heat the sur- 
face of our globe, which is communicated 
to the immediate atmosphere ; it is through 
this fact that the temperature is highest 
Where the place is so situated as to receive 
with most effect the rays of the sun, and 
that it varies in each region wi^h the season ; 
it is also the cause why it decreases in pro- 
portion to the height of the air above the 
surface of the earth. The mest perpen- 
dicular rays falling on the globe at the 
equator, there the heat of it is thC greatest, 
and that Iieat decreases gradually to the 
poles, of course the temperature of the air 
is in exact unison ; from this, it appears, that' 
the air acquires the greatest degree of 
warmth over the equator, whence it be- 
comes insensibly cooler till we arrive at the 
poles; in the same manner, the air imme- 
diately above the equator cools gradually. 
Though the temperature sinks as it ap- 
proaches the pole, and is highest at the 
equator, yet as it varies continually with 
the seasons, it is impossible to form an ac- 
curate idea of the progression without form- 
ing a mean temperature for a year, from 
that of the temperature of every degree of 
latitude for every day of the year, which 
may be accomplished by adding together 
the whole of the observations, and dividing 
by their number, when the quotient will be 
the mean temperature for the year. “ The 
diminution,” says Dr. Thomson, “ from the 
pole to the equator takes place in arith- 
metical progression; or to speak more pro- 
perly the annual temperature of all the 
VOI,. IV. 
latitudes, are arithmetical means betvveen 
the mean annual temperature of the equator 
and the pole.” Mr. Mayer has the honour 
of this discovery, but Mr. Kirwan rendered 
it more simple and plain, by founding an 
equation on it, by which he calculated the 
annual mean temperature of every degree 
of latitude between the equator and the 
pole; the following was the principle of 
proceeding. ‘‘ Let the meah annual heat 
at the equator be m, and at the pole m — n ; 
put O for any other latitude; the mean 
annual temperature of that latitude will be 
m — X sin. If therefore the tem- 
perature of any two latitudes be known, 
the value of m and n may be found. Now 
the temperature of north lat. 40° has been 
found by the best observations to be 62.1°, 
and that of lat. 50°, .52.9°. The square of 
the sine of 40° is nearly 0.419, and the 
square of the sine of 50° is nearly 0.586. 
Therefore, 
m — 0.41 n =z 62.1, and 
m — 0.58 n =: 52.9, therefore 
62.1 -f 0.41 n = 52.9 -f 0.58 n as 
each of them, from the two equations is 
equal to m. From this last equation the 
value of n is found to be 53 nearly; and m 
is nearly equal to 84. The mean tempe- 
rature of the equator, therefore is 84°, and 
that of the pole 31°. To find the mean 
temperature for every other latitude, we 
have only to find 88 arithmetical means 
between 84 and 31.” 
Mr. Kirwan calculated a table of the 
mean annual temperature of the standard 
situation in every latitude, which answers 
only for those ,of the atmosphere of the 
ocean, as it was made for that part of the 
.Atlantic situated betvveen 80° north and 
45° south latitude, extending westw’ard to 
the gulf stream, within a few leagues of 
the American coast ; and for all that part 
of the Pacific Ocean from the 45th degree 
of northern to the 40th of southern latitude, 
from the 20th to the 275th degree of lon- 
gitude east of London. Mr. Kirwan terms 
this part of the ocean the standard, as the 
rest is subject to anomalies to be mention- 
ed liereafter. The same industrious gen- 
tleman ascertained the monthly mean, tem- 
perature of the Standard ocean; that of 
Api il approaches very nearly to tlie annual 
mean, “ and as far as heat depends on the 
action of solar rays, that of each month is 
as the mean altitude of tlie sun, or rather as' 
the sine of the sun’s altitude. 'Phe learned 
investigators to whom we are indebted for 
these experiments and observations, £®v. 
