Let AB reprefent thé feftipn óf’a cylinder -moveable
aboutpits axis, which is parallel to the horizon '; at the
end there is an index 1 moveable again® a, graduated arc.
ab ; Shout this, cylinder.fame catgut m i is wound; one,
end of which is fixed ito.the cylinder,..and the otherrend
to fomeflung; immoveable atyZ.i Now aj the moifture
of the air incfeafiS; the oatgut contraéts and turns.the
cylinder, and the motion .of the index ffiows the increafe
o f the- moifturey-aq_d as the air decreafes in moifture,
the catgut will lengthen, and the weight of the index
will cany thé cylinder back, and'the index will (how
the correfpMamg decreafc. of moifture., -
306. |^ fd e i?\to-.make a perfeft hygrometer,^ fuch
fpbftapcës mull be ufed that will cpntraft or expand in
proportion to. The quantity of moifture received. Mr.
D b Luo.has made agrértmany experiments in order to
find out fuch 'iuEfianSsf and. -the refult is, that
whalebone and box,--ctU acrofs’ the" fibres, increafe very
nearly, in proportion to the quantity of moifture received..
He preferred the whalebone, firft, on. account of its.
ftëadinefs, in alvvaysvlcqmiii^^o '-the fame point at extreme
moiftore.;' fecoijdly, on account of its greater ex-f
panfion, it increafiiig inJength abovê onie'ejghth of it-
felf, fropn extreme .‘dryni-fs to extreme moiuiire’; laftly,
•it is.mórë epfity mndcthin and narrow:' j
- 301. 11k 8 A Ussuri-: and D e Luie have proved by
,the hydrometer, that the air increafes.in drynêfs as you
afeend m the atmosphere; fo that in the upper attain-
able'regipns, it is'confta'nfly very ’dry, except in the
clouds. The forajer gentleman has alfo Jhown, that, if
the whole atmofphere pafled front extreme drynefs to
.extréme moifture, the quantity of water thus evaporated
would not raife the barometer half .an inch. Laftly,
:in-chemical operations on the ai£$he greateft quantity1
opf evaporated water .that may be fuppofed in them at the
xommon temperat-ury of the ratmofphere;-- even if they
were at extreme ig&t&me, is not fo much as- the one
hundredth part .of their .raaf?.
On the Afcent of ‘Vapours, the 'D right of
=■ : Springs; and Formation of Rain, Smwmhd
302. Vapours are-raifed from the füfface o f the 'moift
earth and waters-jfithc principal caufe o f whichfis, pro -
Uably, the heat o f the .fun, the evaporation being always
greateft wheni-Tw bijat ’is" greateft.- ‘T he ff iffic uR ’or
folving: the'phWii^'ehon arifes from., hence, that w e find
a heavier SuitflfivatJer) fufpe’nded in a.lightér-ijtii33(air),
CoiiLravy ter the common principle o f hydroftaHcs.
303. D r . Halley fi.ppofe'cf, that by thelia'^ipii'.hf
the fun up'ori the Surface o f the water, the aqueous particles
become mi$iFa fie to hull i v M'fi'Blesf'filleil with
warm and' rarefied ai fn as to make the whole bulhfpe-
. lightéKThau air, in 'whiéfccafethl particles will
afeend.. 13ut 'tlicfc.is great difficulty in conceiving hpw
•this can Ij'cift:flffiff i|(d. , A n Irtf ibub’))?,. a? f r j l
tiiu iMb'rmed, ||vhenf they 'afeend;. Ol.eyaii withi 1 would
foqq;.be rcouj.i5l tixlFu rifrrc*teir j eratuve: o f the air u ilr-
•sfut, qn^luEh ac'coqhf they wiilld lntmedi ïtély defcènd;
T h e m o t probable fuppófition' is, Uiatt'dynp’bratibn is a
chemical folir! ion of wafeifin':'aif. W( know th t metals
are diflolved in menftruums, aiid'f'tbjeir particles diffufed
.and fnfjiciuicd in the fluid, although the 1 ' he grayit
y be greater than that bfSè® fluid. Heat promotes,
this folution; in the day time then fort the heat <1 ïifësia
jnöte ,p*^iBiipïqlüti(^_thap what. cangiaké '.place ingfK’è
night when -the air is colder; in which cafe, the water
'fills in dews and Togs... T h e yapours; fthus /raifed by
heat, afcei.id'i ito tin c old regie 1 < f the atmofphere, and,
not being, there kept in a.ftate o f perfedtfplution1 form
clouds.
.. 3 ° 4" ' Id A k,-fuppbfedh. Spzingsfty. hegxwipg tp-c
xSigwater and mélted^ifiio.w, Which pencil . je ■ 41 fur- -
faces of'hills,. and running by the. fidi ST clay orfrocks.
'which if .cannot penetrate, at laft^b':mrraiptqjf0me''.place
where it: breaks oil!:. This would acccn'nt: for the phaf..
nomenon, provided the fupjily .from thefe; Gahfes; was
fufficient: ■ Now D f . HAfchÉY.:h^;|difcpvered a caufe
fufficient for a s u p p ly fo r h c Kas proved b y experiment,
that the vap.om-s which are raifed, afford a" much greater
fupply than is ncccflary. W e will give the accimiifTin
his oriVn .wqtds. .• *
395- “ We took a pan ó f water '(falted to tj|edegree
o f tlie.ffaltn^fs o f the feaj;' .by. a folution, o f .about a
fortfefhi part o f faitj^bout 4 inches deep,:a:n^ y ? 4..inolfes
diametei i in which" wé placed a thermometer; and .by
means' o f a pan o f coals,' wé brought the water to tile'
fame degree o f heat which is obferved to be that'of 'tlie
air in our liotteft fuinincrs; the thermometer nicely
Showing i t . 1 This done, we affixed the pan o f water,
.with the thermometer in it, to one end of, the. beam
-of the feales, and exaétly counterjtoifed it'with weights
.at the other end; and by the application or removal o f -
the pan o f coals, we found’ it very eafy to maintain the
water in the fame degree pf heat precisely. Doing tins,
we found the weight of-the water fenfibly to' decreafe ;
and A t the end .of two hours tve obferved, that'there
wanted
■ wanted h alf an o u n e e it» y all but
o f water/'wnich ip” that time nad^gfcpiepff m ’ vapopr z
though o.he could hardly perceive it finoke, and the
water was not-'fenfibly warm, s Thi quantity in fo 'most
a time feerned very confiderable, being 'little lefs thaiisb
ounces in 24 hours, from fo fmall a circle as S. iiicdres ’
di u eter. T o re duee thi exj ei 1 e t to an e a£t r il
ru.Hi/’afidl»g»; d e t e ^ r t i a j ' f ^ f .
v. t e f nliich hid ev'apofiKd, 1 , idumc tie , cxp eii-nct
sflilege l :by D.r. E dw. B krnaro to ha i been made in
i f e f i x f i ’d Society, viz. that! he cubic foot: ttngljh o f water
weighs exaftly 76 pounds 5. y 1728,
the number o f inches’ in. a fobt; will give 253'- grains,
or half an ounce 13 ' ' grail s for '.the weight'of a cubic
Si h ( £ w.ateif;-.wherefore the ght'o'f 233 g li s
1 1 ., or 35 part, J r a cubic inch of water. Now
the area o f the circle; v ’l fe diameter is 7 ' inches, is
4 , |Si ire i dies,’ by. i the quantity,of
v iter evaporated, viz. | of ii'.i ' - .the' qublrent-f,^^,'
or yg, fliows that the thickuefs o f the water evaporated,
was'; tile 53 d part o f ’in inch : but we will, fu-pp fe it to
be only the S e th part, f i r the fa ility o f .call llatipn
' I f therefiie water, as warm as the air in fummer, erfraTeg
CiS th ic k ufs ol tht fL th p u t o f .11 jh i’ i ,in* If'yy,'
hours1,from its whole furface, in 12 hours it 1 iH exhale
-/-g o f an inch ; which quantity will be found abundantly
fufficie | to fe e i r all the rains, fpr ng ,, and de s,!
and account fori the Caf£ iti Sr 1’ being always it a
iftan 3, neithei w lfti lg nor overflow! 1 g ; as likewife’ for
the current: .laid to fet always in at the ftraights o f Gib'
raltar, though thofe ‘Mediterranean Seat -receive fo
many, and fo confiderable rivers. 5?
-': 306. T 0 eftimate the quanti y o f water arifing in
vapours'out o f the fea, I think I ou.glfi to eohfidei it
only tlie.'.time tfie.fun is upf-fpr^thap thefdews return in
the nrght as’ rhucli,. if not inore .vappursithaW are then
-emitted 1 and in fummer the -days bt ing longer than 12
teJiiEj, tins excefsfis'balanced by tile wi ikei ray 5 o f the
fun,,especially when ^ riling, before .the water is warmed :
£o th'dt"if l lallbw' ,’^ q f ah inch*of i l . i fiiffu e of the lta
to Be raifed'per diem in vapours, i f Tpay nc^ h'e. an irripro-'
'Uabfe cohjefTbre.
397. Upon this fupporfition, every 10 Iquart 'iiiih J
-of the furface o f the water, yields in vapour, per diem,
a cubic inch o f water j and every fquafc foot, half a
<vyine pint'; every fpace o f 4 Square feet, a gallon ; a
mile Square, 6914 tons ; a Square degree, fuppofe o f 69
Mnglifb miles, will evaporate 33 million of.tons;. and i f
•thS Mediterranean be eftimated at 40 degrees lpngjiand
4. broad, allowances being made'for the places whei=e'it
is broader byptliofe that are narrower, (and I am fine I
guefs at the leaft,) there will h e 160- fquare degrees o f
Tea ; and eqUfequentlyThe whole Mediterranean Sea mull
lofe in.vapour, in a rummer’ s day, at leaft 5280 milhons
loTtons. A n d this quantity * ofpyapoui', though very
great, is as little as can be" concluded frimi the cxp.erii.
ment produced : and yet there remains another fea life,
■ which cannot be reduced to the rule, I meanfthe winds,
■ whereby the furface .of .tlie water is licked
•vWi.. ftwhat
fafter than it exhales b y the heat o f the fun, as it
is weft’ known to thofe that have cpnfidered thofe d r y - ,
ing winds vyhich blow Sometimes. I
308. The Mediterranean receives ’ thefe confiderable
rivers : the Iberus, the Rhone, the Tiber,, the Po, the
Danube, the Isfiejler, t'he lioryjlhencs, the Tanais, and the
'Nile, .all £ 1 being o f no great note, and: the®
quantity o f water inconfiderable. We will fuppofe each
o f .thefe;nine rivers to; bring down, ten times as much
water, as the . fiver Thames, not that any o f them is fo
.great in- reality,.but to, comprehend with them 'all the
fmall rivulets that, fall into the Fea, which otherwise I
kh()hpnpt ;hoiv to allow fo r ..
3:09. T o calculate the water of the Thames, I a flume
that "at’KsMlfjLj(Bridge, where the flood never reaches;
and the water always rnnt down, the breadth o f the
chanhelpis roo yards, and its -depth 3, it being reduced
fo an equality (infipth which fuppofifiohs I am fire I
take the- 'thoft). Hence, the profilq o f the water in
this place, is 300 fquare yards (.'this multiplied by 48
■ miles, (which X allow the water to run in 24 hours, at’
‘2 miles in an hour) or' S/p^So yards,, gives 25344000
cubic yards of water to be evacuated every day, that is,
20300.000 tons per diem ; and I d’p'ub.t hot but in the
excefs’iof ifiy meafire o f the channel o f the river, I have
made, more than fufficient allowance for the waters o f
the Brent', the iWandel, the Lea, and the Derwent, which
are all worth notice, th a t'fa ll into the Thames below
H f f i H r a S p p M ■
» 3 10-. Noyv i f each o f the afprefaid nme rivers; yield
ten times; | ssmuch water asi the Thames doth,, it will foh
low,-that; each „ o f them yields but ,'203. millions o f tons
per die)/!, and the whole nine but 1827 millions of tons
in a day ; which is but little more than y o f what is
.raifed. by vapours out o f the Mediterranean in twelve
.3x1. Thus the Doctor has Ihown that the waters
raifed by vapours are vaftly more than fufficient fo^ the
fupply o f all’iftle ■ rivers ; the overplus may fall, partly
upon the fea, and. partly upon the flat lands, and not
contribute to f'lJl.fne.pivers. We may therefore admit
Mr. M a r r io t te ’ s folution o f the caufe o f fprings.
■ 312. B'efides the conjlant fprings, there are others
which ebb andfonv alternately, which may be thus ac-
counted for. ..The water, .before it breaks out, may meet
with a large cavity on theffidie o f the hill, and upon the
overflowing o f this refervoir, it may find an -aperture,
and make its efcape.; in cafe o f dry weather,- therefore,.-
the fupply o f water, may. not, be fufficient to keep it fill®
in which cafe;, the fpring wili.ceafe to. flow, and continue
dry, till a fupply caufes.it to overflow, and produce
again the fpring. ,
313. There is another.theory to account fpr fprings
and rivers; which 'refers this caufe to a great a’oyfs o f
waters occupying the, central parts o f pui- .globe. . I t
aflerls, that all the( .phenomena o f fprings are chiefly
idh-'iyed fibiii' .the-, vapours, veins, and iffiues, o f this great
^®pfs, into, which .they are returned ; and that a perpetu
al ■ circ’ulation.'iand -equality is kept up; 'the Tp'rings
f W W B l f B never