73« BAROMETER. 
thought of placing the mercury at top, and the water at 
bottom, which lie thus contrived. ADG (fig. 3) is a 
bent tube hermetically fealed at. A, but open at G, of 
about one line in diameter, and palling through tire two 
equal cylindrical vell'els B C, EF, which are about twenty 
inches apart, and of fifteen lines diameter, their length 
being ten. The mercury, being put into the tube, will 
ffund between the middle of the veffels E F and B C, the 
remaining fpace to A being void both of air and mercury. 
Laftly, common water, tinged with a fixth part of aqua 
regis, to prevent its freezing, is poured into the tube F G, 
till it riles a foot above the mercury .in D F. To prevent 
the waterfront evaporating, a drop of oil of fvveet almonds 
floats on the top of it. But the column of water will be 
fenlibly affected by heat and cold, which l'poils the accu¬ 
racy of the inftrument. 
The Horizontal or Rectangular Barometer, 
reprefented in fig. 4, was invented by J. Bernoulli and 
Caffini -f -where All is a wide cylindrical part at the top 
of the tube, which tube is bent at right angles at C, the 
lower part of it C D being turned into the horizontal direc¬ 
tion, and clofed above at A, but open at the lower end D, 
where the mercury cannot run out, becaufe it is oppofed 
by the preffure of the atmofphere. This and the forego¬ 
ing contrivance of Huygens are founded on the theorem 
in hydroftatics, that fluids of the fame bale prefs accord¬ 
ing to their perpendicular altitude, not according to the 
quantity of their matter ; fo that tlie fame preffure of the 
atmofphere fuftains the quickfilver that fills the tube A 
G D and the cittern B, as would fupport the mercury in 
the tube alone. Hence, having fixed upon the lize of the 
fcale, fuppofe twelve inches, in dead of three, in the com¬ 
mon barometer from twenty-eight to thirty-one, that is 
four times as long ; then the area of a feition of the cylin¬ 
der A B mult he four times that of the tube, and confe- 
quently its diameter double, lince the areas of circles are 
as the fquares of their diameters : then for every natural 
variation of an inch in the cylinder A B, there will be a 
variation of four inches in the tube C D. But, on account 
of the attrition of the mercury againfi the tides of the glafs, 
cmd the great momentum from the quick motion in C D, 
the quickfilver is apt to break, and the rife and fall is no 
longer equable ; and befides, the mercury is apt to be 
thrown out of the orifice at D by any Hidden blow or mo¬ 
tion of the machine. 
The Diagonal Barometer of Sir Samuel More¬ 
land, is (hewn at fig. 5, and is another method of enlarg¬ 
ing the natural fcale of three inches perpendicular, or C 
D, by extending it to any length, as B C, in an oblique 
direction. This is liable in foine degree to the fame in¬ 
convenience, from friction and breaking, as the horizon¬ 
tal one ; and hence it is found that the diagonal part'B C 
cannot properly be bent from the perpendicular more than 
in an angle of 43 0 , which only increafes the fcale nearly in 
the proportion of feven to five. 
Doctor Hook’s Wheel Barometer, fig , 6, was 
invented about 1668, and is meant to render the alterations 
in the air more fenlible. Here the barometer tube has a 
large ball A B at top, and is bent up at the lower or open 
end, where an iron ball G floats on the top of the mer¬ 
cury in the tube, to which is connected another ball H by 
a cord, hanging freely over a pulley, turning an index K 
L about its centre. When the mercury rifes in the part 
F G, it raifes the ball, and the other ball defeends, and 
turns the pulley with the index round a graduated circle 
from N towards M and P; and (lie contrary way when the 
quickfilver and the ball link in the bent part of the tube. 
Hence the fcale is ealily enlarged ten or twelve fold, being 
increafed in proportion of the axis of the pulley to the 
length of the index K L. But then the fritftion of the 
pulley and axis is fome obftruction to the free motion of 
the quickfilver. 
The Steelyard Barometer, for fo that may be 
called which i reprefented bv fig. 7, which enlarges the 
fettle i;i the proportion of the fliorter to the longer arm of 
a fteelyard. A B is the barometer tube, clofc at A and 
open at B, immerfed in a cylindrical glafs cittern C D, 
which is but very little wider than the tube A B. The 
barometer tube is fufpended to the fhortcr arm of the in¬ 
dex like a Iteelyard, moving on the fulcrum E, and the 
extremity of its longer arm pointing to the divifions of a 
graduated arch, with which index the tube is nearly in 
equilibrio. When the preffure of the atmofphere is lei - 
fened, the mercury defeends out of the tube into the cif- 
tern which raifes the tube and the Ihorter arm of the in¬ 
dex, and confequently the extremity of the longer moves 
downwards, and pafles over a part of the graduated arch. 
And on the contrary this moves upwards when the preffure 
of the atmofphere increafes. 
The Pendant Barometer, fig. 8, was invented by 
M. Amontons, in 1695. it conlifts of a llngle conical 
tube A B, hung up by a thread, the larger or open end 
downwards, and having no veffel or ciftern, becaufe the 
conical figure fupplies that, and the column of mercury 
fuftained is always equal to that in the common barometer 
tube; which is effected thus: when the preffure of the air 
is lefs, the mercury (inks down to a lower and wider part 
of the tube, and confequently the altitude of its column 
will be lefs ; and on the contrary, by a greater preffure of 
the atmofphere the mercury is "forced up to a higher and 
narrower part, till the length of thp column C D be equal 
to that in the tube of the common barometer. The incon¬ 
venience of this barometer is, that as the bore mult be 
made very fmall, to prevent the mercury from falling out 
by an accidental (hake, the fvktion and adhefion to the fides 
of the tube prevent the free motion of the mercury. 
Mr. Rowning’s Compound Barometers, have 
feveral contrivances for enlarging the fcale in any proporo 
tion whatever. One of thefe is deferibed in the Philof. 
Tranf. No. 427, and alfo in his Nat. Philof. part 2, which 
is here reprefented at fig. 9. A B C is a compound tube, 
hermetically fealed at A, and open at C ; empty from A 
to D, filled with mercury from thence to B, and from 
hence to E with water. Then, by varying the proportions 
of the twb tubes A F and F C, the Icale of variation may 
be changed in any degree. 
The Marine Barometer. This was firft invented 
by Dr. Hook, to be ufed at fea, being contrived fo as noi 
to be affected or injured by the motion of the (hip. His 
contrivance confided of a double thermometer, or a couple 
of tubes half filled with fpirit of wine ; the one fealed ai> 
both ends, with a quantity of air included ; the other feal¬ 
ed at one end only. The former of thefe is a fleeted only 
by the warmth of the air; but the other is affected both 
by the external warmth and by the variable preffure of the 
atmofphere. Hence, confidering the fpirit thermometer 
as a ftandard, the excels of the rife or fall of the other 
above it will (hew the increafe or decreafe of the preffure 
of the atmofphere. This inftrument is deferibed by Dr. 
Halley, in the Philof. Tranf. No. 269, where he fays of it, 
“ I had one of thefe barometers with me in my late fouth- 
ern voyage, and it never failed to prognofticate and give 
early .notice of all the bad weather we had, fo that I de¬ 
pended thereon, and made provifion accordingly ; and from 
my own experience I conclude, that a more lifeful contri¬ 
vance hath not for this long time been offered for the be¬ 
nefit of navigation.” 
Mr. Nairne, an ingenious artift in London, has lately 
invented a new kind of Marine Barometer; which differs 
from the .common barometer by having the lower part of 
the tube, for about two feet long, made very fmall, to 
check the vibrations of the mercury, which is apt to arife 
from the motions of the fliip. This is alfo aflifted by be¬ 
ing hung in gimbals, by a part which lubjefts it to be the 
lead affected by fuch motions. Another fort of Marine 
Barometer has alfo been invented by M. Paffemente, an 
ingenious artift at Paris. This contrivance conlifts in twill¬ 
ing the middle of the tube into a fpiral of two revolutions; 
by which the impulfes the mercury receives from the (hip’s 
motions are deftroyed, by being directed contrariwife. 
3 The 
