MANOMETER. 
will not aft truly, except it be in a vertical pofition ; and 
even then it is neceflary to give it a fmail degree of mo¬ 
tion, to bring the quickfilver into its true place ; where it 
will remain in equilibrio, between the exterior preffure of 
the atmofphere on one fide, and the interior elaftic force 
of the confined air on the other. 
“ Pounded ice and water were ufed to fix a freezing 
point on the tube 5 and, by means of fait and ice, the air 
was farther condenfed, generally four, and fometimes five 
or fix, degrees below o. The thermometer and manometer 
were then placed in a tin veflel among water which was 
brought into violent ebullition ; where, having remained 
a fufficient time, and motion being given.to the manome¬ 
ter, a boiling point was marked thereon. After this the 
fire was removed ; and the gradual descents of the piece 
of quickfilver, correfponding to every twenty degrees of 
temperature in the thermometer, were fuccefiively marked 
on a deal rod applied to the manometer. It is to be ob- 
ferved, that both inftruments, while in the water, were in 
circumftances perfectly fimilar; that is to fay, the ball 
and bulb were at the bottom^f the veflel. In order to 
be certain that no air had efcaped by the fide of the quick¬ 
filver during the operation, the manometer was frequently 
placed a fecond time in melting ice. If the barometer 
had not altered between the beginning and end of the ex¬ 
periment, the quickfilver always became ftationary at or 
near the firft mark. If any fudden change had taken 
place in the weight of the atmofphere during that inter¬ 
val, the fame was noted, and allowance made for it in af¬ 
terwards proportioning the fpaces. 
“ Long tubes, with bores truly cylindrical, or of any 
uniform figure, are fcarcely ever met with. Such how¬ 
ever as were ufed in thefe experiments, generally tapered 
in a pretty regular manner from one end to the other. 
When the bulb was downwards, and the tube narrowed 
that way, the column of quickfilver confining the air 
lengthened in the lower half of the fcale, and augmented 
the preffure above the mean. In the upper half, the co¬ 
lumn being Shortened, the preffure was diininiflied below 
the mean. In this cafe, the obferved fpaces both ways 
from the centre were diminifhed in the inverfe ratio of 
the heights of the barometer at each fpace, compared with 
its mean height. If the bore widened towards the bulb 
when downwards, the obferved fpaces, each way from the 
centre, were augmented in the fame inverfe ratio; but in 
the experiments on air lefs denfe than the atmofphere, the 
bulb being upwards, the fame equation was applied with 
contrary figns; and, if any extraordinary irregularity 
took place in the tube, the correfponding fpaces were pro¬ 
portioned both ways from that point, whether high or 
low, that anfwered to the mean. 
“The obferved and equated manometrical fpaces being 
thus laid down on the pafteboard containing the meafures 
of the tube; the aia° of the thermometer, in exaft pro¬ 
portion to the feftions of the bore, .were conftrufted 
alongfide of them ; hence the coincidence with each other 
were eafily feen ; and the number of thermo,metrical de¬ 
grees anfwering to each manometrical fpace readily tranf- 
ferred into a table prepared for thepurpofe.” 
The linking peculiarity of manometers of the above 
conftruftion, and that on which their chief excellence de¬ 
pends, is that a mercurial column of about -j^th or^th of 
an inch in diameter Hides freely up and down a glafs tube, 
without fuffering any air to pafs either way. This cha- 
rafter is, however, obtained only by preferving the tube 
and mercury very clear and dry. If any dull, moifture, 
or oxyd, be found in the tube, the mercury becomes lefs 
free in its motion 5 and the air is apt to break the mercu¬ 
rial column, and gradually efcape. A bora of lefs diame¬ 
ter would occaljon too much friction, and one of greater 
would fuffer the mercury to fall down. 
When the expanfion or dilatation of the air in any ex¬ 
periment amounts to one-half of the original volume, or 
any other quantity exceeding that, a manometer of a ftill 
more Ample conftruftion may be ufed, namely, a ftraight 
Von, XIV. No. 975/ 
297 
tube, or one without bulb, of the fame bore or capacity 
as Col. Roy’s. It mull be divided into equal fpaces, by 
means of a Aiding mercurial column, on account of the 
irregularity of the bore incident to fuch tubes; a final 1 
drop of mercury may them be let down by a clean iron 
wire to any part of the tube, fo as to conftitute a Aiding 
column of about half an inch in length. 
Another fpecies of manometer may be ufed when the 
objeft is to ineafure the force of fteam or vapour, gene¬ 
rated over certain liquids by heat. In this cafe a tube 
fimilar to the preceding may be bent into a fiphon with 
parallel legs, the AiOrter leg of which muft be clofed, or 
hermetically fealed, and the longer open. A few drops of 
the liquid inuft be conveyed to the extremity of the clofed 
leg; after which the greater part of the tube may. be filled 
with mercury, fo as to leave no fpace with air between 
the mercury and the liquid ; the manometer muft then be 
put into water, &c. of a known temperature, and held in 
a perpendicular pofture, with the bending lowelt, and fo 
that the extremity of the tube containing the liquid may 
be wholly immerfed in the warm water, whilli the other 
extremity is without. The heat will expand part of the 
liquid into fteam, which will deprefs the mercury in the 
fame leg, and elevate it in the oppofite, till an equilibrium 
of preffure is ettablifhed, The elaftic force of the fteam 
will evidently be equal to the preffure of the atmofphere 
± the difference of the heights of the two mercurial co¬ 
lumns in the fiphon, according as the column in the open 
or clofed leg exceeds that of the other. If the difference 
of the heights is expefted to be upwards of thirty inches, 
fome inconvenience arifes from the great length of tube 
requifite; in this cafe an ingenious contrivance has been 
invented to obviate it; the open end of the manometer 
muft be hermetically fealed, fo as to inclofe a column of 
atmofpheric air of due volume ; when the fteam is formed. 
in the liquid, and the mercury depreffed, it condenfes the 
air in the other leg; and the fpace occupied by the con-' 
denfed air, as is well known, is inverfely as the force j 
then the quantity of this force thus afcertained, rfc the 
difference of the two mercurial columns, will give the 
whole elaftic force of the fteam. Great care muft be taken 
that the air-column of the fiphon is dear of the liquid 
that generates the fteam. By this fort of inftrument 
Mr. Dalton finds the force of (team from fulphuric ether 
at zi2° Fahr. = 236 inches of mercury. See Manchefter 
Mem. vol. v. p. 567, Alfo, New Syftefn of Chemillry^ 
p. 14. 
Sauflure, in his Effays on Hygrometry, defcribes his 
manometer; it was nothing but an ordinary barometer ; 
a Ample Itraight tube was filled with boiled mercury, and 
its open end was immerfed in a cup of the fame liquid 5 
the whole was then inclofed in a large glafs balloon, ex¬ 
cept a few inches of the upper extremity of the tube, to 
which a fcale of degrees or equal parts was attached, to 
fnow the variation of the altitude of the mercury. The 
tube paffed through a circular hole in a tin plate which 
covered the opening of the balloon, and which was very 
carefully luted, as was the paffage of the tube, fo as to be 
perfeftly air-tight. In this cafe it was evident the inftru¬ 
ment was no longer a barometer, as it was cut off from the 
aftion of the air out of the balloon ; but the mercury was 
fupported by the Ipring or elafticity of the air within the 
balloon, and muft be fubjeft to fuch fluctuations as took 
place in it, independently of any change of weight in the 
atmofphere. By means of this apparatus, Sauffure found 
that atmofpheric air, in pafling from extreme drynefs to- 
extreme moifture, in the temperature of 65° Fahr. in- 
creafed about -g^th in elafticity ; and vice ver/d, in pafling* 
from extreme moifture to extreme drynefs, it diminilhed 
•jVh In ds elaftic force, the temperature being all she time 
uniform. For the important ui'es of thefe and fimilar in¬ 
ftruments, fee “ Of the Barometer in meafuring Alti¬ 
tudes,” vcl. ii. p. 741, 
Manometer for the air-pump. This is the name given 
to a kind of gauge for meafuring the degree of the rare¬ 
ft G - faction 
