Encyclopaedia Britannica, or, a Dictionary of arts, sciences, and miscellaneous literature : enlarged and improved. Illustrated with nearly six hundred engravings > Volume 13, MAT-MIC

712
Tenoera-
ture of
the \ ir.
METEOROLOGY.
Latit.
Jan.
Feb.
March
April
May
une
July
Auef.
Sept.
oa.
Nov.
Dec.
23
t)8.
72.
75-
7?-9
78.5
79-
79-
79-
78-5
75-
74-
71-
22
69.
72-5
75-5
76.5
79-5
79-5
79-5
79-5
79-
75-5
74-5
7I-5
21'
20
71*
74*
76.
77.2
80.
80.
80.
80.
79-5
77-
75-
72.
72.
74-
77-
77.8
180.5
180.5
iSo.?
80.5
:8o.
78.
75*5
72-5
72.5
76.
77-5
78.3
81.
81.5
81.5
81.5
81.
79-
76.
73-
18° 170 16°
73-
76.5
78.
78.9
81.5
82.
82.
82.
81.5
80.
77-
74-
173-5
77-
78.5
79-4
82.
82.5
82.5
82.5
82.
81.
78.
75-
74- 174-5
77-5 78.
79- 79-5
79-9 80.4
82.5 83.
83-
S3-
S3-
82.5
81.5
78-5
75-5
83.5
83.3
83-5
83-
82.
79-
76.
75-
78- 5
80.
80.8
S3-
83.8
83.8
83.8
3-
2-5
79- 5
76.5
76.
79-
80.8
81.3
83-5
84.
84.
84.
83-5
83-
80.
77-
12
76- 5
79-5
81.
81.7
84.
84-3
84-3
84-3
84.
83-5
80.5
77- 5
77-
79.8
81.5
82.
84.
84.6
84.6
84.6
84.3
83.8
80.8
78.
ioL
77-5
80.
81.8
82.3
84-3
84.8
-4.8
84.8
84.6
84.
T.
8.5
Chap. II,
i empera-
ture of
the Air.
if It appears from the above table that January is the
coldeft month in every latitude 5 that July is the warm-
eft month in all latitudes above 48° j that in lower
latitudes Auguft is generally the warmeft month ;
that the difference between the hotted: and coldeft
months increafes according to the diftance of the place
from the equator. All habitable latitudes are found
to enjoy a medium heat of 6o° for at leaft 2 months,
which is a very favourable circumftance, as probably no
corn could be produced under a lower medium tempera¬
ture. The temperatures within io° of the poles differ
very little, nor do they differ much within io° of the
equator. Hence it was unneceffary t© note thefe lati¬
tudes in the table. The temperatures of different years
vary but little near the equator, but this difference in¬
creafes more and more as the latitudes approach the
poles.
Tempera- It is well known that the temperature of the at-
ture de- mofphere gradually diminifhes according to the height
V* i And ^ie place above the level of the fea. It was found
in the air. by Dr Hutton of Edinburgh, that a thermometer kept
on the top of Arthur’s feat, a height of about Soo feet,
ufually flood 30 lower than one kept at the foot of this
hill; and Bougucr obferved that on the top of Pinchin-
cha, a height of about 15564 feet, a thermometer flood
540 lower than it did at the level of the fea in the fame
latitude.
We are indebted to Dr Kirwan for a very ingenious
method of determining the rate of the diminution in the
temperature in particular cafes, having the temperature
of the furface of the earth given. The temperature of
the atmofphere conftantly diminilhing as we rife above
the level of the fea, we muft at a certain height arrive
at a point where a perpetual congelation takes place.
This point muft vary in height according to the la¬
titude, being higheft at the equator, and coming gra¬
dually nearer the earth as we approach the poles; it
muft vary aifo with the feafon, being higheft in fummer, jg
and loweft in winter. The cold on the top of Pin- Term of*
chincha was found by M. Bouguer to extend from 70 perpetual
to 9® below the freezing point every morning juft c?nSe^a"
before funrife j hence he concluded that between thetlon'
tropics the medium height of the term of congelation
(where it freezes at fome part of the day all the year
round) Ihould be fixed at 15577 ^eet above the level
of the fea ; but in latitude 28°, and during the fummer,
at 13440 feet. If we take the difference between the
temperature at the equator, and the freezing point, this
difference will bear the lame proportion to the term of
congelation at the equator, thatthe difference between the
medium temperature at any other latitude and the freez¬
ing point bears to the term of congelation at that lati¬
tude. Suppofe the medium heat at the equator to be
84°, the difference between which and 3 2° is 520 ; and
fuppofe the medium heat of latitude 28° to be 72T^°,
the difterence between which and 3 2° is 40°Ty>* Then
by the following proportion, 52015577“ 40^5 : 12,72°
gives us the term of congelation at 28°. In this way
Dr Kirwan proceeded in calculating the following
tional to the mean altitude of the fun, or rather to the fine of that altitude. If, therefore, we have the mean
heat of April, and the fine of the fun’s altitude given, the mean heat of May may be found by the following pro¬
portion :
As the fine of the fun’s mean altitude in April : the mean heat of April s: the line of the fun’s mean altitude
in May : mean heat of May,
In the fame manner the mean heat of June, July, and Auguft may be found $ but for the temperature of the*
fucceeding months we muft take into confideration another circumftance, linc« the above rule would make the
temperature of thefe months too low, as it does not take in the heat derived from the earth, which is nearly equal
to the mean annual temperature. The real mean beats of thefe months muft be confidered as an arithmetical
mean between the aftronomical and terreftrial heats. Thus, for latitude 51°, the aftronomica! heat of September
being 44.6°, and the mean annual heat 52.4®, the real heat of September ought to be — 48.5. Dr
Kirwan, however, after going through a tedious calculation, found the refults to correfpond fo little with aflu&l
obfervation, that he drew up the table partly from calculating from principles, and partly from an examination of
feveral fea journals.