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ftvnaod, as well as theii- d>;feaive principle, renders tliem 

 mere mechanical piaurcs, and not fcientilic inlb-umeiits, m 

 the parlour. 



An inllrumcnt of this kind, with confiderable improve- 

 ments, has been conllrudtd by Mr. Fitzgerald, F. R. S. 

 It is furnidied with two pulleys that move on friftion- 

 wheels ; each of which turns an index on the centre of a 

 graduated circle. The fmaller circle is four inches in 

 diameter, and divided into three equal parts, each of 

 which is again fubdividcd decimally ; and the chano:es 

 correfponding to tlic rile or fall of the mercury from 28 

 to 31 inches, are marked on the margin of it, as they are 

 on the fcalts of common barometers. The larger circle is 

 divided into 300 cciual parts ; and being about 30 inches 

 in circumference, the index belonging to it will mark dif- 

 tinftly the 600th part of an inch in the rife or fall of the 

 mercury. On the centre of this circle two resjifters are 

 fixed, which are placed along the index when the inftru- 

 ment is adjufted ; one of them is carried round with the 

 index, and left behind on its return ; fo that their diftance 

 will determine the limits of the variation from one obferva- 

 tion to another. Phil. Tranf. vol. lii. part i. N° 29. Ibid. 

 voi.lx. N°io. 



The pemhint barometer, invented by M. Amontons, in 

 1695, is a machine rather pretty and curious than ufeful 

 (fg. S3). It confilts of a conical tube, placed vertically, 

 its upper and fmaller extreme hermetically fealed ; it has 

 Tio vcffel or ciilern, its conical figure fupplying that defedt : 

 for when filled, like the reft, there will be as much mercury 

 fuftained as is equivalent to the weight of the atmofphere ; 

 and as that varies, the fame mercury takes up a different 

 part of the tube, and fo becomes of a different weight. 



Thus, when the weight of the atmofphere is increafed, 

 the mercury is driven up into a narrower part of the tube ; 

 by which means its column is lengthened, and, for the 

 reafon jull given, its weight increafed. Again, the at- 

 mofphere decrcafing, the mercury finks into a wider part of 

 the tube ; by which means its column is again (hortened, 

 and its preffure accordingly weakened. Thus, the fame 

 mercury is ftill a balance to the atmofphere under all its 

 variations. The inconvenience in this barometer is, that 

 to prevent the mercury and air from changing places, the 

 bore of the tube mull be very fmall ; which fmallnefs of 

 the bore renders the friftion fo fenfible as to impede its 

 playing. 



The marine barometer is a contrivance of Dr. Hooke, in 

 1700, to be ufed at fea, where the motion of the waves 

 renders the others impraaicable ; it refembles that of Amon- 

 tons invented in 1705. This is no'Jiing more than a double 

 thermometer, or a couple of tubes half filled with fpirit of 

 wine ; the one hermetically fealed at both ends, with a 

 quantity of common air inclofcd ; the other fealed at one 

 end, and open at the other. 



Now the air, we know, is able to aft on the fpirit of 

 wine, and raife it, two ways ; partly by its gravity, as 

 in the Torricellian tube ; and partly by its heat, as in the 

 thern-omcter. If then the two tubes be graduated, fo as 

 to agree with each other at the time when the air is inclofed, 

 it will eafiiy fallow, that, wherever the two agree after- 

 wards, the preffure of tlie atmofphere is the fame as at the 

 time when the air was inclofed. If in the thermometer 

 open to the air the liquor ftand higher, confidering at the 

 fame time how much the other is rifen or fallen from the 

 other caufe of heat or cold, the air is heavier ; on the con- 

 traiy, when it is lower, compared with the other, the air is 

 lighter than at the time when the inftrument was graduated. 

 Here the fpaces anfwering to an inch of mercury will be 



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greater or lefs, according to the quantity of the air inclofed, 

 and the fmallnefs of the tubes ; and they may be increafed 

 almoft in any proportion. But it muff be remembered, that 

 the denfity and rarity of the air, on which this machine is 

 founded, do not only depend on the weight of the atmo- 

 fphere, but alio on the aftion of heat and cold. This, 

 therefore, can never be a juft barometer ; but may properly 

 enough be called a manofcope, or inftrument to (hew the 

 denfity of the air. See Manometer. 



Nevertheltfs, the inftrument is faid to be of good ufe in 

 giving notice of all bad weather at fea, as alfo of veerable 

 winds, and of the neighbourhood of ice. Phil. Tranf. 

 N^ 429. p. 133. 



Improved marine larometirs. In the beft of thefe baro- 

 meters, Meffrs. W. and S. Jones apply a fmall ivory floating 

 gage, or index, to an aperture in the ciftcrn of mercury be- 

 low ; tlie index floats on the mercury ; a mark is cut on its 

 ftem, and another on the focket in which it moves ; thefe 

 two marks are brought to a coincidence by turning the fcrew 

 below : and thus the furface of the mercury in the ciftern 

 is made to be juft to the divifions of the plate above. 



Mr. Nairne, an ingenious artift in London, conftrufted a 

 marine barometer for captain Phipps, in his voyage to the 

 north pole ; the upper part of which was a glafs tube, 

 about three tenths of an inch in diameter, and four inches 

 long, to which another glafs tube was joined with a bore 

 about -j^th of an inch diameter. Thele two glafs tubes 

 formed the tube of this barometer, which was filled with 

 mercury, and inverted into a ciilern of the fame. The in- 

 ftrument was fixed in gimbals, and kept in a perpendicular 

 polition by a weight faftened to the bottom of it, and was 

 not liable to the inconvenience attending the common ba- 

 rometer at fea. Voyage to the North Pole, p. 123. 



The marine barometer, as it is commonly conftrufted, dif- 

 fers from the common one merely in having the bore of the 

 tube fmall for about two feet in its lower part ; but above 

 that height it is enlarged to the common fize. Through 

 the fmall part of the inftrument the mercury is prevented 

 from afcending too haftily by the m.otion of the Ihip, and 

 the motion of the mercury in the upper wide part is con- 

 fequently lefftned. Much depends upon the proper fnfpen- 

 fion of this inftrument : and Mr. Nairne has found by ex- 

 periment the point from which it may be fulpended fo as 

 not to be affefted by the motion of the fhip. 



We (hall here fubjoin the defcription of two kinds of 

 marine barometers, which are conftrufted by Meffrs. W. and 

 S. Jones of London, and which feem to be well adapted to 

 marine purpofes. In Plate Y^. fg.%6. one of thefe barometers 

 is reprefented as fupported on its ftand in the cabin of a fnip, 

 ready for obfervation : a h c are the folding mahogany legs, 

 about three feet each in length ; ^ is a circular brafs plate, 

 with two hollow brafs tubes fixed perpendicularly upon it ; 

 a gimbal brafs ring with its axis is made to turn between 

 thefe tubes ; and on two fpiral fprings placed in the tubes, 

 the axis of the gimbal ring afts. The barometer frame B 

 is attached inwards to this ring by an axis and two fcrews, 

 in a pofition at right angles to the axis in the uprights, yet 

 left free to move ; the three legs are fcrewed down to the 

 floor of the cabin. Whatever heave or motion the ffiip 

 may receive, the barometer, by its aft:on on the gimbal, on 

 the fprings in the tubes, and on its axis, will always tend 

 to keep its vertical pjfition, and as fpeedily as poffible attain 

 to a ftate of quiefcence ; f/ is a fcrew that ferves to move 

 the Aiding Nonius fcale upon the plate above ; ^ is a fmall 

 mahogany door that is fliut over the tube and plate, to de- 

 fend them when this inftrument is not in ufe. On the top of 

 the frame there is a pendent brafs ring g, by which the ba- 

 rometer. 



