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

„ • ,5
Ilainy days
often more
numerous in
'winter.
71B M E T E O
^rnnTnd ^lc contral7> the number of rainy days is fmall-
Kaiii at efluat°r» and incrtafes in proportion to the
< diftance from it. From N. Lat. 1 20 to 430 the mean
number of rainy days is 78 ; from 430 to 46° the mean
number is 103 5 from 46° to 50° it is 134 ; from 510
to 6o°, 161 days.
I he number of rainy days is often greater in winter
than in fummer y but the quantity ot rain is oreater m
fummer than in winter. At Peterfburgh, the number
of rainy or fnowy days during winter is 84, and the
quantity which falls is only about 5 inches ; during
fummer the number of rainy days is nearly the fame&
but the quantity which falls is about 11 inches.
More rain falls in mountainous countries than in
plains. Among the Andes it is faid to rain almoft
perpetually, while in Egypt it fcarcely ever rains at all.
If a rain-gauge be placed on the ground, and another
at fome height perpendicularly above it, more rain will
be colle&ed into the lower than into the higher ; a
proof that the quantity of rain increafes as it defeends,
owing perhaps to the drops attra&ing vapour during
their paiTage through the lower ftrata of the atmofphere
where the greatell quantity refides. This, however,
is not always the cafe, as Mr Copland of Dumfries
difeovered in the courfe of his experiments. He ob-
ferved alfo, that when the quantity of rain colleaed in
the lower gauge was greateft, the rain commonly con¬
tinued for fome time ; and that the greatell; quantity
■was collefled in the higher gauge only either at the end
of great rains, or during rains which did not halt long.
Thefe obfervations are important, and may, if followed
out, give us new knowledge of the caufes of rain.
I hey feem to Ihow, that during rain the atmofphere
is fomehow or other brought into a Hate which induces
it to part with its moifture 5 and that the rain continues
as long as this Hate continues. Were a fufficient
number of obfervations made on this fubjedl in different
places, and were the atmofphere carefully analyfed du¬
ring dry weather, during rain, and immediately after
rain, we might foon perhaps difeover the true theory of
rain.
Rain falls in all feafons of the year, at all times of
day than in the daf> and durir>g the night as well as the day j
the night, though, according to M. loaldo, a greater quantity
falls during thesday than the night. The caufe of rain,
then, whatever it may be, mutt be fomething which
operates at all times and feafons. Rain falls alfo du¬
ring the continuance of every wind, but ofteneft when
the wind blows from the fouth. Falls of rain often
37 happen likewife during perfedt calms.
Mean an- It appears from a paper published by M. Cotte in
thy'T^n the Jrj“rna/ de Physique (or Odlober 1791, containing
m <>o r» n 11 f-»Fir 11 ... _ a. ■« —
36
More rain
fa 1 is in the
R ° L ° G Y- Chap. m.
bable therefore, if the imperfeaion of our data warrant
any conclufion, that fome of the vapour is adlually de- tioil and
compofed in the atmofphere, and converted into oxygen llain-
and hydrogen gas. J 6 '
The dry land amounts to 52,745,253 fquare miles :
the quantity of ram falling on it annually therefore
will amount to 30,960 cubic miles. The quantity of
water running annually into the Iba is 13,140 cubic
miles ; a quantity of water equal to which muft be fup-
plied by evaporation from the fea, otherwife the land
would foon be completely drained of its mofture.
.The quantity of rain falling annually in Great Bri¬
tain may be feen from the following table :
:ity 01 ram ^ / . r ... ~ ty-i —““““***!
in (ireat the mean quantity of rain falling at 147 places, fitua-
Ikitain ted between N. Lat. ii° and 6o°, deduced from tables
kept at thefe places, that the mean annual quantity of
rain falling in all thefe places is 34.7 inches. Let us
fuppofe then (which cannot be very far from the truth),
that the mean annual quantity of rain for the whole is
34 inches. The fuperficies of the globe confifts of
170,981,01 2 fquare miles, or 686,401,498,47 1,475,200
fooare inches. The quantity of ram, therefore, falling
annually will amount to 23,337^50,81 2,030,156,800
cubic inches, or fomewhat more than 91,751 cubic miles
of water. This is ■ 6.191 cubic miles of water lefs
than ihe quantity of water evaporated. It feems pro-
Years of
obfervation.
8
8
45
5
8
18
7
5
H
10
5
5
20
8
Places.
Rain in
Inches.
Dover
Ware, Hertfordfhire
London
Kimbolton
Lyndon
Chatfworth, Derbyfhire
Mancheiler
Liverpool
Lancatier
Kendal
Dumfries
Branxholm, 44 miles S. W. of
Berwick
Langholm
Dalkeith
G lafgow
Hawkhill
Mean
37-52
23.6
17-5
23-9
22.21
27.865
43-i
34-41
4°-3
61.223
36.127
3r-26
36.73
25.124
31-
28.966
32.532
Mr Dalton has eftimated the quantity of rain that
falls in England at 21 inches j but as no account is ta¬
ken of what falls in Wales and Scotland, this eftimate
probably falls much fhort of the real annual quantity.
In this country it generally rains lefs in March than in
November, in the proportion at a medium of 7 to 12.
It generally rains lefs in April than Oaober, in the
proportion c>f 1 to 2 nearly at a medium. It generally
rains lefs in May than September j the chances that
it does fo are at leaf! as 4 to 3 : but when it rains plen¬
tifully in May, it generally rains but little in Septem¬
ber ; and when it rains one inch or lefs in May, it rains
plentifully in September.
The degree of moifture that is prefent in the atmo¬
fphere at any given time, is meafured by the hygrome¬
ter. Under the article Hygrometer we have amply
deicribed feveral of the moft important inftruments of
that kind; but there is one hygrometer, viz. that of
My Lefiie, which remains to be delcnbed in this place.
Figures of the inftrument are given in Plate CCLXXVI.
%• I3, I4-
T he principal part of the inftrument is compofed of Leflie,8s hy-
two glafs tubes terminated by hollow balls, one tranf-grometer.
parent and the other opaque. The tubes are fele&ed, as
regular as. poffible, from 4 to 8 inches long, and about
of an inch thick, or as {lender as thofe employed
for