BAR (303) BAR mercury be fufained in the tube by the preffure thefe are the limits between which it always ftands of the atmosphere, by carring it to a higher fi- in this country, on the common furface of the tuation, it would defcend lower in the tube, ha- earth Then meafare, from the furface of the ving a fhorter column of the atmosphere to fu- quickfilver in the bafon at F, 28 inches to K, and tain it, and vice verf. And Pafcal engaged his 31 inches to I, dividing the space between them brother-in-law, M. Perier, to try that experiment into inches and tenths, which are marked on a for him, being more conveniently fituated for that fcale placed against the fide of the tube; and the purpofe than he was at Paris. This he accord-tenths are fubdivided into hundredth parts of an ingly executed, by obferving the height of the inch by a fliding index carrying a vernier or noniquickfilver, in the tube, first at the bottoni of a mountain in Auvergne, and then in feveral differ- vided, will anfwer for all the ordinary purposes US. Thefe 3 inches, between 28 and 31, fo dient affitudes; wherein it was found that the mer- of a stationary or chamber barometer; but for excury fell lower and lower all the way to the top periments on altitudes and depths, it is proper of the mountain; and fo confirming the truth of to have the divifions carried on a little higher up, the doctrine relating to the univerfal preffure of and a great deal lower. In the proper filling and the atmosphere, and the confequent fufpention otherwife fitting up of the barometer, feveral cirof the mercury in the tube of the barometer. cumftances are to be carefully noted: as, that Thus, by the united endeavours of Torricelli, the bore of the tube be pretty wide, to allow the Defcartes, Pafcal, Merfenne, Huygens, and others, the caufe of the fufpenfion of the quick fil- peded by an adhesion to the fides; that the bafon freer motion of the quickfilver, without being imver in the tube of the barometer became pretty below it be alfo pretty large, in order that the generally established. It was fometime, however, furface of the mercury at F may not fenfibly rife after this general confent before it was known that the preffure of the air was various at differ- the tube be cut off rather obliquely, that when it or fall with that in the tube; that the bottom of ent times at the fame place. This could not, refts on the bottom of the bafon there may be a however, remain long unknown. The frequent free paffage for the quickfilver; and that, to have meafuring of the column of mercury foon thewed the quickfilver very pure, it is beft to boil it in its variations in altitude; and experience and ob- the tube, which will expel all the air from it. fervation taught that thofe variations in the mer- This barometer is commonly fitted up in a neat curial column were always fuccceded by certain mahogany cafe, together with a thermometer and changes in the weather, as to rain, wind, frofts, hygrometer, as reprefented in Plate XXXVI. fig. &c. Hence this inftrument foon came into ufe, 2. As the fcale of variation is finall, being includas the means of foretelling the changes of the weather; and on this account it obtained the name ed within 3 inches in the common barometer, seof the weather-glass, as it did that of barometer the fcale, or to render the motion of the quickveral contrivances have been devifed to enlarge from its meafuring the weight or preffure of the filver more fenfible. Defcartes first fuggefted a meair. cuted by Huygens This was effected by making thod of increafing the fenfibility, which was exethe barometrical tube end in a pretty large cylin drical veffel at top, into which was inferted alfo the lower or open end of a much finer tube than the former, which was partly filled with water, to give little obftruction by its weight to the mo tion of the mercury, while it moved through a pretty long fpace of the very fine tube by a fmall variation of the mercury below it, and fo rendered the fmall changes in the ftate of the air very fenfible. But the inconvenience was, that the air contained in the water gradually difengaged itself, and efcaped into the vacuum in the top of the fmall tube, till it was collected in a body there, and by its elafticity preventing the free rife of the fluids in the tubes, fpoiled the inftrument as a barometer. And this is the reafon why a water barometer cannot fucceed. This barometer is reprefented in fig. 3. CD is the veffel, in which is united the upper or fmall water tube AC, with the lower or mercurial one CB. To remedy this inconvenience, Huygens thought of placing the mercury at top, and the water at bottom, which he thus contrived. ADG, fig. 4. is a bent tube hermetically fealed at A, but open at G, of about one line in diameter, and palling through the two equal cylindrical veffels BC, EF, which are about 25 inches apart, and of 15 lines diameter, their length being 10. will stand between the middle of the veffels LF The mercury being put into the tube. and BC, the remaining space to A being vold Loth (3.) BAROMETER, COMMON, invented by Torricelli, is reprefented Plate XXXVI. fig. 1. AB is a glafs tube, of, or, or inch wide, (the wider the better,) and about 34 inches long, being close at the top A, and the open end B immerfed in a bafon of quickfilver CD, which is the better the wider it is. To fill this, or any other barometer, take a clean new glafs tube, of the dimenfions as above, and pour into it well purified quick filver, with a fmall funnel either of glafs or paper, in a fine continued ftream, till it wants about half an inch or an inch of being full; then stopping it clofe with the finger, invert it flowly, and the air in the empty part will afcend gradually to the other end, collecting into itself fuch other small air bubbles as unavoidably get into the tube among the mercury, in filling it with the funnel: and thus continue to invert it feveral times, turning the two ends alternately upwards, till all the air bubbles are collected, and brought up to the open end of the tube, and till the part filled fhall appear, without speck, like a fine polified fteel rod. This done, pour in a little more quickfilver, to fill the empty part quite full, and fo exclude all air from the tube; then ftopping the orifice again with the finger, invert the tube, and immerte the finger and end, thus ftopped, into a bafon of purified quickfilver. In this pofitica withdraw the finger, fo fhall the mercury defcend in the tube to fome place as C, between 48 and 31 inches above that in the bafon at F, as of air and mercury. Laftly, common water, tinged with a 6th part of aqua regia, to prevent its freezing, is poured into the tube FG, till it rifes a foot above the mercury in DF. To prevent the water from evaporating, a drop of oil of fweet almonds floats on the top of it. But the column of water will be fenfibly affected by heat and cold. which fpoils the accuracy of the inftrument. Although the invention of this barometer was claim. ed by Huygens and alfo by De la Hire, it appears firft to have been contrived by Dr Hooke in 1668, and defcribed in Phil. Tranf. N° 185. It is moft delicately moveable, and when properly managed, by far the fitteft for a chamber, for amufement, by obfervations on the changes of the atmospheric preffure. The flighteft breeze caufes it to rife and fall, and it is continually in motion. But this and all other inftruments of the kind are inferior to the common barometer, for all philofophical purposes both on account of their being lefs manageable, and alfo in point of accuracy. For their scale must be determined in all its parts, by the common barometer, and therefore notwithstanding their great range they are fufceptible of no greater accuracy than that with which the scale of a common barometer can be observed and measured. Our attention should therefore be entirely directed to its improvement and portability. Its fcale fhould be moveable, so as to admit of being brought to the furface of the mercury in the ciftern; or the ciftern fhould be capable of being brought to the beginning of the fcale, and the inftrument fhould be furnished with magnifying glaffes, to obferve the coincidence of the fcale with the mercury, at the top, and at the 'ciftern. (4.) BAROMETER, DIAGONAL, invented by Sir Samuel Moreland, fig. 5. is another method of enlarging the natural feale of 3 inches perpendicular, CD, by extending it to any length BC in an oblique direction. This is liable in fome de. gree to the inconvenience of friction and breaking; and hence it is found that the diagonal part BC cannot properly be bent from the perpendicular more than in an angle of 45°, which only increases the fcale nearly in the proportion of 7 to 5. (5.) BAROMETER, DR HOOK'S WHEEL, fig. 6. This was invented about 1668, and is meant to render the alterations in the air more fenfible. Here the barometer tube has a large ball AB at top, and is bent up at the lower or open end, where an iron ball G floats on the top of the mercury in the tube, to which is connected another ball H by a cord, hanging freely over a puley, turning an index KL about its centre. When the mercury rifes in the part FG, it raifes the ball, and the other ball defcends and turns the pulley with the index round a graduated circle from N towards M and P; and the contrary way when the quickfilver and the ball fink in the bent part of the tube. Hence the fcale is eafily enlarged 10 or 12 fold, being increafed in proportion of the axis of the pulley to the length of the index KL. But then the friction of the pulley and axis is fome obftruction to the free motion of the quickfilver. Contrivances to leffen the friction, &c. may alfo be feen in the Phil. Tranf. vol. 52. art. 29. and vol. 6o. art. 10. (6.) BAROMETER, HORIZONTAL, or RECTAN GULAR, was invented by J. Bernoulli and Caflini; and is reprefented in Fig. 7. A B is a pretty wide cylindrical part at the top of the tube, which is bent at right angles at C, the lower part of it CD being turned into the horizontal direction, and clofe at A, but open at the lower end D, where however the mercury cannot run out, being there oppofed by the preffure of the atmofphere. This and the foregoing contrivance of Huygens (See 3.) are founded on the theorem of hydrostatics, that fluids of the fame base press according to their perpendicular altitude, not according to the quantity of their matter; so that the fame preffure of the atmosphere fultains the quickfilver that fills the tube ACD, and the cif tern B, as would fupport the mercury in the tube alone. Hence, having fixed upon the fize of the fcale, as fuppofe the extent of 12 inches, instead of the 3, in the common barometer from 28 to 31, that is four times as long; then the area of a fection of the cylinder AB must be 4 times that of the tube, and confequently its diameter double, fince the areas of circles are as the fquares of their diameters: then for every natural variation of an inch in the cylinder AB, there will be a variation of four inches in the tube CD.-But on account of the attrition of the mercury against the fides of the glafs, and the great momentum from the quick motion in CD, 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 fudden motions of the machine. (7) BAROMETER, MARINE. This was firft invented by Dr Hooke, to be ufed on board of fhips, being contrived fo as not to be affected or injured by the motion of the fhip. His contrivance confifted of a double thermometer, or a couple of tubes half filled with fpirit of wine; the one fealed at both ends, with a quantity of air included; the other fealed at one end only. The former of thefe is affected only by the warmth of the air; but the other is affected both by the ex ternal warmth and by the variable preffure of the atmosphere. Hence, confidering the fpirit ther mometer as a ftandard, the excefs of the rife or fall of the other above, it will fhew the increase or decrcafe of the preffure of the atmosphere. This inftrument is defcribed by Dr Halley, in the Fl lf. Trans. N° 269, where he fays of it, "I had one of thefe barometers with me in my late fouthern voyage, and it never failed to prognofticate and give early notice of all the bad weather we bad, fo that I depended thercon, and made provifion accordingly; and from my own experience I conclude, that a more cfeful contrivance bath not for this long time been offered for the benefit of navigation." Mr Nairne, an ingenious arti in London, has lately invented a new kind of MaFine Barometer: which differs from the common barometer by having the lower part of the tube, for about two feet long, made very mall, to check the vibrations of the mercury, which would otherwife arife from the motions of the thip. This is alfo affifted by being hung in gimbals, by a part which fubjects it to be the leaft affected by fuch motions. Another fort of Maine Baron ter has also been invented by M. Paffemente, an ingenious artist at Paris. This contrivance confifts only in twisting the middle of the tube into a fpiral of two revolutions: by which contrivance the impulfes which the mercury receives from the motions of the fhip are destroyed, by being tranf. mitted in contrary directions. (8.) BAROMETER, MR BOYLE'S STATICAL. This barofcope confifts of a large glafs bubble, blown very thin, and then balanced by a fmall brafs weight. Hence these 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 atmosphere becomes denfer, the ball lofes more of its weight, and the brass weight preponderates; and contrariwife when the air grows lighter. : (9.) BAROMETER, MR CASWELL'S. This barofcope feems to be the most fenfible and exact of any; and is thus defcribed in the Philof. Tranf. Vol. 24. Suppofe ABCD, fig. 8. is a bucket of water, in which is the barofcope xreyzof my which confifts of a body x r sm, and a tube eyzo, which are both concave cylinders, made of tin, or rather glafs, and communicating with cach 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 zu. To the top is added a small concave cylinder, or pipe, to keep the inftrument from finking down to the bottom: md is a wire: and ms, de are two threads oblique to the furface of the water, which perform the office of diagonals for while the inftrument finks more or leis by an alteration in the gravity of the air, where the furface of the water cuts the thread is formed a fmall bubble, which afcends up the thread while the mercury of the common baroope afcends, and vice verfa. It appears from a caiculation which the author makes, that this infrument fhews the alterations in the air 1200 times more accurately than the common barometer. He obferves, that the bubble is feldom known to fland 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 paffing over it always makes it defcend, &c. While fome have been increasing the fenfibility of the barometer, by increafing the variations, others Lave endeavoured to make it more convenient by reducing the length of the tube. M. Amontons, in 1688, first propofed this alteration in the structure of barometers, by joining feveral tubes to one another, alternately filled with mercury and with air, or fome other fluid; and the number of thefe tubes may be increased at pleafure: but the contrivance is perhaps more ingenious than ufeful. (10.) BAROMETER, MR MAIRAN'S REDUCED, is only 3 inches long, and ferves the purpose of a manometer, in showing the dilatations of the air in the receiver of an air pump. Inftruments of this kind are now commonly applied to this use. (11.) BAROMETER, MR ROWNING'S COMPOUND. This gentleman has feveral contrivances VOL. III. PART 1. for enlarging the fcale, and that in any proportion whatever. One of thefe is defcribed in the Philof. Tranf. N° 427, and alfo in his Nat. Philof. part 2; and another in the fame part, which is reprefented Plate XXXVI. fig. 9. ABC 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 wa ter. Hence by varying the proportions of the two tubes AF and FC, the fçale of variation may be changed in any degree. (12.) BAROMETER, NEW SPECIES OF. Among the various kinds of barometers and barofcopes, may be ranked, a new fort of Barometer, or WEA THER INSTRUMINT by the Sound of a WIRE. This is mentioned by M. Lazowski in his Tour through Switzerland; it is yet but in an imperfect state, and was lately difcovered there by accident. A clergyman, who was near fighted, often amufed himfelf with firing at a mark, and contrived to stretch a wire fo as to draw the mark to him to fee how he had aimed. He obferved that the wire fometimes founded as if it vibrated like a mufical chord ; and that after fuch foundings, a change always enfued in the ftate of the atmosphere; fion whence he came to predict rain or fine weather. On making farther experiments, it was found that the founds were moft diftinct when extended in the plane of the meridian. According to the wea ther which was to follow, the founds were more ar lefs foft, or more or left continued Fine wea ther was announced by the tones of counter-tenor, and rain by thofe of bafs. It has been faid that M. Volta mounted 15 chords in this way at Pavia, to bring this method to fome precition, but no accounts have yet appeared of the fuccefs' of his obfervations. (13.) BAROMETER, PENDANT, fig. ro, was in-. vented by M. Amontons, in 1695. It confifts of a fingle conical tube AB, hung up by a thread, the larger or open end downwards, and having no veffel or ciftern, because 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 finks 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 atmo phere the mercury is forced up to a higher and narrower part, till the length of the column CD be equal to that in the tube of the common barometer.--The inconvenience of this barometer is, that as the bore must be made very finall, to prevent the mercury from falling out by an accidental fhake, the friction and adhelion to the fides of the tube prevent the free motion of the mercury. (14.) BAROMETER, PORTABLE, is to 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 micr cury; which being preffed by the air forces the mercury into the tube, and keeps i fufpended at its proper height. This bag is ufually inclufed in a box, through the bottom of which pattes & fcrew, by means of which th mercury may br forced up to the top of the tube, and prevented from |