neVer failing, and the fea, by reyfon o f its communication
with thefub'terranêouswàters, "never overflowing.
In finking mines: and wells from ,8 to 800 feet deep, it
is common to break in upon powerful fources o f water,
and tbefe fometimes at very great depths., Springs near
the furface may'have their fources from refervoirs which
lie deeper-, and they in their turns are fed by larger and
deeper, till we come to the grand repofitory o f all,
which & fuppofed to keep up a communication wiLh the
fea, in confequence o f which, the water in the earth has
always.a tendency to rife to the level o f the fea. Dr.
D erhart has ihown, that fprings occur in great plenty,
arid are confiant in their courfe, even in times o f the
greateft drought, where the country is in general very
low, and there are no mountain tops to condenfe the
vapours. M . G uAlteri fays, that the waters dif-
charged b y the rivers in Italy into the fea, are to the
rain which falls upon the land, as 55 to 27. T he earth
is alfo moiftened to' a greater depth than can be accounted'
for from the falling o f the rain. From all
thefe circumftarices it is concluded,- that there iriuft be
fubterraneous refervoirs o f water. I t is not Unlikely
h u t that this, and the caufe ó f fprings arid rivers alfigned
b y Dr. H a l l e y , may both operate.
314. Clouds are formed by the water railed b y evaporation,
and are o f the fame nature as dews and fogs'
upon the earth. When the water in the air ceafekto
be fufpended, i t falls down, and the particles Uniting in
falling, form drops. Various, probably, are the caufes o f
the precipitation o f the water. - A f te r the air is fatura-
t e iw i th vapour, a hidden diminution o f the denfity
o f the air will caufe it to part with fomê o f its vapours j
for as a certain quantity o f air can hold but a certain
quantity o f water in foration, i f that air become rarefied,
it will not hold all its water in folution, and a precipitation
make it probable that it is produced by the fame caufe
- as?" when i t is attended by , thundpr -and. lightning^
L ig h t has been Teen amongft -ffieoiou4| hy^riight. in
' rainy ■ ^ea'therj and"even b y .day, rainy ,cjuuds'aréTsen
to have a brightnfcfs evidently independent o f fn£ A n -
T h é intenfity. o f ekftrie ity alfo in hfs apparatus^ nïuallÿ
correfponded very well with the quantity o f rain. T lie
phænomena alfo p t' tHunderf lightning, and; rainj are
very frequentlyobferved to accompany each othei, which,
ihows the connexion they have wit^à^qmnMm^p^üfei
He'ffippôîës that previo'us to rpin, %, efu^ptify pfjqleAric
matter efbap'es' oflt o f the earth, and lSi its afeent, colle
As à quantity *ôf vapour, and thus the air, .heçgmes
overcharged with vapours. Hence, the! rain wil^he
heavitf t je more vigorous £he eleAricity Wf-anaAkis is
agreeable to obfervation. Mr. de L u c MKfljbwn that
water in a ftate o f vanout combo‘ d‘ w ilh îh e an, p iq -
ducesuowmoiftn'efs, and thcrefwe’ qdnçrades tlia tra iji'
ddes.ho,t àrife'fçooi the moiftjire1 in the!'atmofpheré prior
to the rain. T he decompofition produces the luaiftïïre
and then ihefra'm. I f it b'e yery cedd .in thefe régions
'where t ie rain begins to be- fo rm ed .it then defeenas in»
fnow ; affd’when the ardps.of raihraie’' Formed, and are
defeending, i f 'iç their defcfcn? they, para ‘through, a region
will take place. A s vapour is principally^
railed b y beat, a variation o f temperature will probably"
caufe a precipitation. A lfo , we know b y an eleArome-
ter, that the air is always in a ftate o f eleAricitys ibind-'*
times pofitive, and fometimes negative. . From M. de
S I ussure’ s obfervations, in winter the eleAricity
was generally weakeft in an evening, when the dew had
fallen, until the fun’ s "riling ; it afterwards increaféd,"
and generally before noon i t attained its maximum, and
then dimmiihed, till the fall o f the dew, when it would
he fometimes iironger than it had been dUririg the whole
day ; after which, it would gradually diminiih the
whole night. In fummer, in general,-if the ground
have been dry for fome days, and the air been dry alfo,
the eleAricity generally increafes from the riling o f the
-fun till 3' or 4 in the afternoon, when it is ftrongeft ;
f t then diminiihes till the dew begins:to fall, and then it
increafes ; but after this it 'declines, and is very fmall
during the night. Now B e c c a r ia .reckons rain, hail,
and fnow, amongft the efteAs o f the eleAricity o f the
atmofpliere. Clouds which bring rain, he thinks, are
produced in the fame manner as thunder-cl'ouds are,
only b y a lefs degree o f eleAricity. H e remarks feveral
circumdances attending rain without lightning, which
o f the nir cold'enough .to freeze them, they défcçnd
in hail.
On the Temperature * a f different Fa rts of the
Earth , i
’ 315. T he pretence ôf'thé finris' gije ô f tHS principal
fources of'heat, arid its: àbfenee'thb caufe-’b feo ld ’; and,
were theft the only lource o f heat ind i f hi,' in tin fame 1
parallel o f latitude there wpiild“ be the deg! ic o f
heat or c o ld a tth e fam a f afuii.f f, it tint i f fc,i.n lrntliei
contrary ta matter o f faA 5 the temperature o fth e caff-,
'brn! edaft !i>f North America i- ninth colder than (the4
weftefn "coaft o f l-'urope, u n ^ r Vcryhc>
t days.are frequently felt in thc'dîiîdéft 1 uni^tes : a* I,
vety cpld * weather, eyéu’ pprpotual^fribwipsî fouud’ jn .
countries under the equators.. "Wo ’rfiuft thettfoie leek
for other_caufes of-heat and ccid, and tihefe mttft evî-
dently.be. partly local.
316. One great fdurce Of ’Kekt is frocnthé. earth 1
whether this antes from any central* firé,for'from à mate’
o f heal diffufed through the earth, it'is nrit perhaps eafy-
to fay ; the latter caufe is perhaps the moft probable ; -
and in this cafe, the heat which is thus gradually loft
is'renewed again b y the fun. This heat imparted from
the earth to the atmofpliere, tends greatly to moderate
.the feverity.of the winter’ s cold. I t i s found b y obfer-’
vation, that the,famé degree o f heat refidcs in all fubterraneous
places at the fame depth, varying a little at
different depths, but is never lefs than 36°-©? F ahrenh
e it ’ s thermometer. There is howev.er an exception
to this in mines, where there is probably fome chemical '
operations going forwards. Mr. K irwan , in his EJti-
matepf the Temperature of different Latitudes, and to whom
we are principally indebted for what we (hall here give
upon
•upon this fubjéft,|obferves, that at' 80 or 90 feet ( f f :
■ »riîîsèdègtli have dnw <5im?$u:ril jilioq 'ivjrlnpfc tram air,!
/Und perhaps, at a i f f tJftM b a ' qtF j f t j i ;
communication) the femperature ô ftlÆjlIwSylttiés’ V eiy
Thus' f nearly kite -farhe’-as -
theTmean-1 anrfual’ ‘temperature, 'and varies-, very little’1-in
îdiff&rèn't.féafoiia. jjfirîK^effip^rat
obftiva'ory at Pari i. 'y b o u f 5 3 1 ‘dfgiefc',’¥ n l'J a iif f j
about liait A deg: ce-iÿr n eH-tcohl'h 1 rs* ; - f dibll h 1 ;.
about 90 feet. ' !,Tbe^|ira||pkl rtb* W A < !
\ climate -is. à 1 w ays - above qu5,h n d Aoeerare^hijinôvw-ë-!'
aferally ’b'egirïslto : melttfirfl?at‘^ ^ ^© ^ fl^ i® C È p|& i,t
fôurce
known thaA-vapbnf.^àfehsk;grerit*Maq&»|©l'|nêal,
whioli pr dîne 10 olhèi nfftet, b it that ot 1 ki ^*rt
aflupie an-aenak ex-pandraffiate, ufitîl thé vapour i’ 't?dng
drnftd ratoa.fitpfid'',ddn «■ 1 ht huiiderda i 1 1 ! cÉrtHih’
quantityrof heatS^fGapeSÿ ahdtw'armsftbèbfriiabrindMiglat-
moiphere. - Thisc'qridêfeïatibn: -is frequently ^ ^ pM fb y
kbn^ttraSâon-ôî api'H 6 ^ n p 4 *d ^ fik a^ |^ K e fe f® t® |
great 1 iltiauel- nhitl \ e tuqu.utl\ e\pciîlncc beforL
rain, and particularly before a thurider-'ftottfr.
î 3 17 . A s the earth.is düe o f t he great fources ofrlieat,
warming the fiurounding ait, diftance fr >m 'tlîe'Ldfth
mull: be a fource o f cold ; arid thus we find thathae ydii
afeend in the atmofphere, the cold u x ! iv ft , In the
vicinity oPParis, the temperature o f stKe earth being47°):
at fh e eftipiated height of(iio84ffee"t it<was. found^tq
he .21 ?» nr 1 1° below congelation, by M. C harles who
afeended in a balloon. A n d Lord MoLGRAVE,' at the
hbttom p f Tiacffyl Rill, fat| 8o°j found the temperature
fp'f the air; jp ° ; but on the to p, at the height o f 1503
'-feet® only 42°. IT'etfoe we find, that the higheft mountains,
even under the equator, have their tops continually
covered with' fnow. Mr. B.ouW er found the cold
o f Line hind, one o f til e-Cordeliei-es, immediately under
the line, to extend from 70 tb 90 ,below .the freezing
point every morning before fun-rife ; and lienee at a
certain tteiglff ^MiM'vaj-ies in almoft every platitude, it
conilantly freezes 2tirhig^.t. all the year round'j though in
the warm climates it thaws'to fome degree the next day.
This height he calls the loauer Term o f congelation : .between
the tropics lie1 place's 'it "a t'the height o f 15577
feet, Englifli meafurc. The next great fource o f cold
is evaporation, i ’The!fame caufe which makes the con-
denfation.of vapouyafource o f heat, makes 'evaporation .
the ‘ fource o f cold'; as it abforbs the fire in the latter
inftancc, which it gives out in the former : the heat, thus
abforbed is called latent heat, it producing, in that ftate,
( no fenfation o f warmth, t A t a certain height1 above the
lower term o f congelation- it never freezes,’ r.ot lucca-.ifi: I
the cold .decreafes, but becaute1 the 'vapours do nor
afeeud. fo h ig h ; this- height Mr. B o u g u e r calls the
itpfef term o f congelation, andmnder the-.equator he fixCs '■
it at the height b f a'SoooTeet. Mr. K irwan has given us
the following mean height o f the upper and: lower terms
o f congelation, for thp latitude o f every five degrees, in
La t.
-^.k. lower 1
’ T e rm ..
' A l t , upper
' ,T u m.’ Lat:.,'
rA.lt.- lower
-iiTfârm'.o. ‘r
j A ] t. upper
■ ' Term. ;
o ° 15.777;. 28000“ : 45s ■ - 7658*- \ mm 5 >5457"'-■ • ^ ü § 50 6260
10 15.0Ó7 - ' 27084 55 4912
*5 r '14498“ '26d6r 6a 3684 Ü M H
-zb'1 ■ . 24661 ‘ 2516 ■ . 4676
■ Mllll 7Ó' i *557' f
B W
10664 '19^09, 80 Iqo ; ;; i H
AO 16207
Sometimes tire temperature o f the upper-aS-iq'
higher than-that o f the-lower air, particularly when a
large mafs.of vapouis is condenfed b y eleArical agency;
for no part o f the heat given OM. by-that caufe being
'.left by communication with air much colder, that which
furrounds the vapours fo condenfed} muft b e heated to
a cotffiderable degree. T h e clouds, by abforbing the
fun’ s rays, are more heated than the clear air would be.
Thefe, and other .circumftances, render the true height
o f the terms ó f congelation at any time, fubjeA to con-
.fiderable uncertainty.
319. The