BAROMETER. 
737 
The Statical Baroscope, or Barometer of Mr. 
Boyle, conlifts of a large glafs bubble, blown very thin, 
and then balanced by a final 1 braf's weight. Hence, thefe 
two bodies being of unequal bulk, the larger will be very 
much affected by a change of the denfity of the medium, 
but the lefs not at all as to fenfe : fo that, “when the atmo- 
Iphere becomes denfer, the ball lofes more of its weight, 
and the brafs weight preponderates; contrariwife when the 
air grows lighter. 
Mr. Caswell’s Baroscope, or Barometer, feems 
to be the molt fenfible and exaft of any. It is thus de- 
fcribed : Suppol’e A 1 * C D, fg. io, is a bucket of water, 
in which is the barofcope a rezyofm, which confifts of a 
body xrfm , and a tube ezyo, which are both concave 
cylinders made of glafs, and communicating with each 
other. The bottom of the tube zy has a leaden weight 
to fink it, fo that the top of the body may juft fwim even 
with the furface of the water by the addition of fome grain 
weights on the top. When the inftrument is forced with 
its mouth downwards, the water afcends into the tube to 
the'height yu. To the top is added a fmall concave cy¬ 
linder, or pipe, to keep the inftrument from finking down 
to the bottom : md is a wire : and m S, de, are two threads 
oblique to the furface of the water, which perform the of¬ 
fice of diagonals : for while the inftrument (inks more or 
lefs by an alteration in the gravity of the air, where the 
furface of the water cuts the thread is formed a fmall bub¬ 
ble, which afcends up thb thread while the mercury of 
the common barofcope afcends, and vice verfa. It appears 
from a calculation which the author makes,- that this in¬ 
ftrument fliews the alterations in the air 1200 times more 
accurately than the common barometer. He oblerves, 
that the bubble is feldom known to ftand ftill even for a 
minute; that a fmall blaft of wind, which cannot be heard 
in a chamber, will fenfibly make it fink ; and that a cloud 
palling over it always makes it defcend, &c. 
While fome have been increafing the fenfibility of the 
barometer by enlarging the variations, others have endea¬ 
voured to make it more convenient by reducing the length 
of the tube. M. Amontons, in 1688, firft propofed this 
alteration in the ftructure of barometers, by joining feve- 
ral to one another, alternately filled with mercury and with 
air, or fome other fluid ; and the number of thefe tubes 
may be increafed at pleafure: but this contrivance per¬ 
haps is more ingenious than ufeful. Marian’s reduced ba¬ 
rometer, which is only three inches long, ferves the pur- 
pofe of a manometer, in (hewing the dilatations of the air 
in the receiver of an air-pump; and inftruments of this 
kind are now commonly applied to this ufe. 
The Portable Barometer is fo contrived that it 
may be carried from one place to another without being 
difordered. The end of the tube is tied up in a leathern 
bag not quite full of mercury ; which being prefled by the 
air, forces the mercury into the tube, and keeps it fuf- 
pended at its proper height. This bag is ufually inclofed 
in a box, through the bottom of which pafles a fcrew, by 
means of which the mercury may be forced up to the top 
of the tube, and prevented from breaking it by dafliing 
againft the top when the inftrument is removed from one 
ftation to another. It feems Mr. Patrick firft made a con¬ 
trivance of this kind : but the portable barometer has re¬ 
ceived various improvements fince; and the mod complete 
of this kind has been defcribed by M. de Luc, in his Re- 
cherches, vol. ii. with the apparatus belonging to it, the 
method of conftrudtion and ufe, and the advantages at¬ 
tending it. Improvements have alfo been fuggefted by 
Sir George Shuckburgh, and Col. Roy, which have been 
carried into execution, with farther improvements, by 
Mr. Ramfden, and other ingenious artifts in London. 
This inftrument is reprefented at Jig. u, inclofed in its 
mahogany cafe by means of three metallic rings a a a. 
This cafe is a hollow cone, fo (haped within as to contain 
fteadily the body of the barometer, and is divided into 
three branches front b to c, forming three legs or fupports 
for the inftrument when obfervations are making, and fuf- 
Vol. II. No. 100. 
taining it at the part d of the cafe, as fhewn in Jig. 12, by 
an improved kind of gimbals, in which its own weight 
renders it fufficiently fteady at any time. In the part of 
the frame fg where the barometer tube appears, is made 
a long flit or opening, that the column of mercury may¬ 
be feen againft the light, and the vernier piece f brought 
down to coincide very nicely with the edge of the mercury. 
When the inftrument is fixed in its ftand, the fcrew / is to 
be turned to let the mercury down.to its proper ftation, 
and a peg at i niuft be loofened, to admit the external air 
to aft upon the mercury contained in the box k. The pro¬ 
per adjuftment, or mode of obferving what is called the 
zero or o divifion of the column of mercury, is by obferv¬ 
ing it in the tranfparent part of the box k, which has a 
glafs tube or refervoir for the quickfilver, and an edged 
piece of metal attached to the external part of it; with 
the edge of which the mercury is to be brought into con¬ 
tact by turning the fcrew l to the right or left as occafion 
. requires. The vernier piece at f which determines the 
altitude of the mercurial column, is firft brought down by 
the hand to a near contact, and then accurately adjufted 
by turning the fcrew e at the top. The divifions annexed 
to the tube of this inftrument may be of any fort, or of 
any degree of fmullnefs, according fo the purpofes it is 
intended to ferve. To accommodate it to the ufe of fo¬ 
reigners as well as natives, there are commonly added 
fcales of both French and Englifli inches, with their fub- 
divifions to any. extent required. Ir is ufual to place the 
French fcale of inches on the right (ide at fg, from nine¬ 
teen to thirty-one inches, meafured from the zero or fur¬ 
face of the mercury in the box k below ; each inch being 
divided into lines or 12th parts, and each line fubdivided 
by the vernier into 10th parts, or 120th parts of inches; 
by means of which the length of the mercurial column 
may be determined to the 120th part of a French inch. 
The other fcale, which is placed on the left fide of the 
inftrument, is divided into Englifti inches, and each indh 
into 20th parts, which by a vernier are fubdivided into 
25th parts, or 500th parts of inches; by this means (hew¬ 
ing the height of the mercury to the 500th part of an Eng- 
lilh inch. But this vernier is figured double, or each di¬ 
vifion is accounted two, which reduces the meafures to 
ioooths of an inch for the conveniendy of calculation, in 
meafuring altitudes of hills, &c. 
A thermometer is always attached to the inftrument, as 
a neceftary appendage to it, being faftened to the body at 
k, and funk into the furface of the frame, to preferve it 
from injury : the degrees of this thermometer are marked 
on two fcales, one on each lide of it, viz. the fcale of 
Fahrenheit, and that of Reaumur ;- the freezing point of 
the former being at 32, and of the latter at o. Alfo on 
the right hand fide of thefe two fcales there is a third, 
called a fcale of corrcElicn, placed oppofite to that of Fah¬ 
renheit, with the words add and JubtraEl marked; which 
fliews the necelfary correction of the obferved altitude of 
the mercury at any given temperature of the air, indicated 
by the thermometer. A new kind of portable barometer for 
meafuring heights, has been lately introduced by the Rev. 
Dr. J. A. Hamilton, of Ireland. Inftead of the leathern bag 
which confines the mercury in the common portable baro¬ 
meter, Dr. Hamilton fubftitutes a cylindrical ciftern of 
ivory, with a fcrewed bottom and an open top, fomewhat 
contraded into a (boulder that receives internally a found, 
clean, and porous, cork, through which the glafs tube is 
nicely inferted and puftied down midway. The conftruc- 
tioii depends on this principle, that fpungy cork affords a 
ready palfage through its pores to the particles of air, but 
prevents the efcape of quickfilver, tinlefs a very powerful 
preflure be applied. The fadf, however, is not precifely 
dated. It is not through the pores of that fubftance, but 
through the minute interftices between the cork and the 
infide of the ivory cylinder, that the air infinuates itfelf. 
Accordingly, fome caution and experience are requifite to 
prevent the (topper from being fitted too tight. Nor can 
the obferver be always allured that the confinement of the 
9 B - cork 
