GA 
swill bea very confiderable difference. When the receiver 
contains no other fluid befides air, then the pear-gage and 
the other gages indicate the fame degree of rarefaction ; but 
if the receiver contain the vapour of water or of other liquor, 
then the pear-gage will indicate a much greater degree of 
rarefaction than the other gages ; becaufe the vapour whicn 
has elafticity fufficient to fupply the place of air in the re- 
e re-admiffion of air, is condenfed into a {pace 
much fmaller than the fame quantity of rarefied air can be 
eondenfed into ; fo that the pear-gage fhews the quantity of 
, i in the receiver; whereas the 
ew the quantity a elaftic fluid which is actually 
receiver. 
cua powers of exhauftion in Mr. Smeaton’s 
air-pump, as they are indicated by his pear-gage, have been 
uated with peculiar attention by Mr. Nairne, F.R.S. 
He was led to profecute a feries of ee amend on this fub- 
je@, by obferving the anaes difference between the in- 
dications of exhauition on —— by this gage, and by 
the common barometer-gage. Having ufed every poffible 
precaution in preparing i different gages, he neverthele ‘ 
found, that, when they are put under a receiver, placed o 
a leather dreffed in alum, and foaked in oil and tallow ac- 
cording to his ufual method, and the pump was worked 
minutes, the quickfilver in the barometer-gage role 
to within one tenth of a 
an inch of the height of the quick- 
filver in the ftandard barometer, which was at that time at 
thirty inches, and indicated that the ai onl 
undred times; wher Ee 
gage indicated a degree of exhauftion equal to fix thoufand 
times; the whole of its cavity, on dipping i its open end into the 
cup of quickfilver, and letting inthe air, except a fix thou- 
fandth part, being filled by the A engines The difference 
of indications by thefe two gages was found eS ore 
confiderable i in fubfequent experiments Havin pp or- 
ava experiments eee the 
> Mr. Smeaton, and others, 
ae accounted oe peal ebferved 
eriments 
a sere ita 
aa that water, whenever the preffure of the dino ie 
nit is diminifhed to a certain nae 18 meer turned 
into vapour, and reduced as fuddenly to water again on re- 
ee the preffure: the degree of preffure varies according 
to the temperature of the water; for when the heat is 72° 
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preffure is reduced to that of three 
i or about one fortieth of the ufual preflure of the 
er turns into vapour. 
vee never the air in the receiver is exhaulted to the above- 
Ynenticned decree, the moifture adhering to the different 
parts of the machine will be converted into an elaftic vapour, 
and fupply the place of the air, which is drawn away by the 
working of the pump ; and the fluid left in the receiver and 
pear-gage will be chiefly this vapour. When the air is let 
into the receiver, the vapour within the pear-gage wi ill be 
reduced to water, and only the real air will remain uncon- 
ch re 
and ‘not 
fring of the saa ded air is diminifhe d; ee ereas the baro- 
meter-gage fhews how much the included fluid is eae 
aoe ean whether it confifts of air or va 
order ie el the truth of this plaufible theo nn Mr. 
ae pee to free every part of his apparatus as much 
rm 
so 
GE. 
as poffible from ae adhering moifture, ee ee that by 
this means he fhould be able to bring tl gages to an 
agreement. Initead of placing the receiver on leather, as 
befo ore, he put it on the pump-plate, made as clean and. 
dry as poffible, and applied a cement round its edge to e 
clude the outward air. When the pump in this ftate was 
worked for ten minutes, the barometer-gage indicated a 
degree of exhauftion nearly equal to fix hn dred ; ‘and, on 
letting the air into the receiver, the pear-gage iadicated a de- 
i hundred alfo. In 
another experiment he put a piece of the “ leather above 
mentioned in the receiver, and found, on working the pump, 
that-the barometer-gage indicated a degree of exhauftion 
of nearly three hundred, but the pe ar-gage indicated a 
egree © exhauttion not lefs than four thoufand. But 
O1 
r 
cluded, in general, that-a canderable quantity of vapour 
arofe from the compound of leather, alum, oil, 
it was his next object to ind out from which of thefe fi 
r 
e quantity of oil a 
it comes from the leather-(ollers. 
weighing a hundred grains: from th efe e periments he found 
that the elaftic vap pow, ah occal fom) fo great a differ- 
ence in the te timony of the gages, arofe chiefly from the 
leat eae and ve 2 from the tallow, oil, and alum. In 
the experiment w a ie is it Pt plied the place of the 
a air fo faft, that he could n n ten minutes make 
the barometer-gage in San a ee ee of exhaultion of more 
eas 
than a bundre and a 3 
25 
Ino 
ofe 
eighty grains, it gai ins in the experi- 
ene and the pear-gage ee a savin only of, two 
hundred and eighty. The leather was afterwards he in 
the fteam of hot water till it had feeaeds its former moif- 
ture and weight, and the degree of exhauftion indicated by 
the pear-gage was a hundred thoufand, and the lofs of 
ght, two grains, as before. In the ial of thefe experi- 
ments, the degree of rarefa@ien indicated by the barometer- 
gage was a hundred and thirty-four ; in the fecond, two 
hundred and fixty -cig 3 in the third, one hundred and 
ferty-feven. The the vapour ariing from {m 
quantities of different f flu ‘ds, an a. i 
taining moifture, was tried ina ariety © of flac and 
having found that the {mall pane, of moifture whi 
haled fromthe fubftances under the receiver, prevente 
pump from exhaufling to a confiderable degree, Mr. Nairne 
fufpected that whenever wet leather had been ufed to con 
net the receiver with the plate, there muit have arifen fo 
great a quantity of vapour as te have prevented the. degree 
of exhaui sia from being near - great as in other initances. 
This led him to another fet 0 1 
certain this faé aving fife placed . 
ump-plate, made cle ean and cry, with only a little oil poured 
oad the outfide edge of it, beth gages agreed in j i 
ararefaction of fix hundred, as before :-but when 
ceiver was fet on leather that had peen toaked for two days 
in water, the rarefaCtion indicated by th 
r -page 
Was 
