BAR 



•f the air is decifively eftablifticd. See Air, JVcighl o/, and 

 Rxperiinsnis ivith the Al K-PiimJ>. 



Notwfthllandiiipf the fatisfaftory diinon ft ration of the 

 air's prefTurc, afforded by the TorrircUian experiment, 

 feme attempts were made by the advocates of a ple- 

 num for evading it, and for explaininpj the phenomena 

 of this experiment by fome other hypothtlis. Accord- 

 ingly Linus contt^nded, that in the upper part of the 

 tube there is a film, or "rope of mercury," wlience his hy- 

 pothefis was called " the funicular hypoihelis," which ex- 

 tended through the feeming vacuity ; and that, by means 

 of thia rope, the rcll of the mercury was fufpended, and 

 kept from defcending into the bifon. In proof of this ab- 

 furd and ridiculous hypothefis he alleged the following ex- 

 periment. Take, fays he, a fmall tube, about 20 inches 

 long, open at both ends; fill it with mercury, and Hop the 

 lower orifice with your thumb. Then clofing the upper 

 end with your finger, immerfe the lower end in ilagnant 

 ir.ercuiy ; and upon the removal of your thumb, there wiil 

 .be a fenfible fuciion of the finger into the tube; and both 

 the tube and mercury will adhere to it fo clofely, that 

 they may thus be carried about the room. Hence he in- 

 fers, that the internal cylinder of mercury in the tube is not 

 fullained by the prtffure of the external air ; for this, he 

 argues, would not account for the ftrong fuftion, and the ad- 

 hefion of the tube to the finger. If the tube be not quite 

 filled with mercury, but a fmall interval of air left at the top, 

 after the tube is immerfed in Ilagnant mercury, a confider- 

 able fuction will be perceived. From thefe experiments, 

 which aCluaUy furnilh evidence of the air's prefliire, the 

 funicular hypotheiis of Linus derived fupport for fome 

 time; but it has been long fince exploded. When it was 

 perceived that the mercur)- on the top of a high mountain 

 fublided, and ftood at a lower height than on a plain, and 

 that in the vacuum of an air pump it defcended to the 

 bottom of the tube, this hypothefis could have no advocates. 

 Hou-ever, an experiment mentioned by Mr. Huygens, in 

 which mercury well purged of its air remained fufpended in 

 a tube at the heiglit of 75 inches, fuggefted a more confider- 

 able difficulty, which has been varioufly folved. See an 

 account of it, under the article Torricellian. For an 

 explication of the phenomenon of a fiphon, which dif- 

 charges water under the exhaufted receiver of an air-pump, 

 fee Siphon. 



BAROMETtR, Common, the Conjlrudtoti of if. — A glafs tube 

 (AB, Plate IX. Pneumaties, fg. 76.) open at one end, and 

 hermetically fealcd at the other A, having its diameter about 

 one third or one fourth of an inch, and its length thirty-thrte 

 or thirty-four inches, is filled with mercury i"o jullly as not 

 to have any air over it, nor any bubbles adhering to the 

 fides of the tube ; which is belt done by means of a fmall 

 paper or glafs funnel, with a capillary tube. If a fmall 

 bubble of air be moved backwards and forwards in the tube, 

 it will help to clear the mercur)' ; which wnll appear, when 

 pure, like a polifted rod of Heel. The orifice of the tube, 

 filled after this manner, fo as to overflow, is then clofely 

 prefied by the finger, fo as to exclude any air between it and 

 the mercury, and thus immerged in a velFel of a convenient 

 diameter, lo however as not to touch the bottom : at the 

 diftance of twenty-eight inches from the furface of the mer- 

 cur)' are fixed two plates, CE and DF, divided into three 

 inches, called " the fcale of variation," and thefe again fub- 

 divided into any number of fmall parts. Laftlv the tube is 

 inclofed in a wooden frame, to prevent its being broken ; 

 the bafon, though open to the air, fecured from dull ; and 

 the barometer is complete. As the lowell ftation of the 

 mercury in this country is about 28 inches, and the higheft 



BAR 



about 3 1 inches above the furface of the mercury in the bs. 

 fon, the former point is the loweft in the fcale ol variation, 

 and in the common barometers, called " weather-glaffcs," 

 it is marked Jlormy ; and the latter is marked on one fide 

 •very dry for the fummer, and on the other very hard frnjl 

 for the winter. To the next half-inch b'elow this highell 

 point are annexed fct fair on the one fide, a.\-\& fet frojl on 

 the other. At the height of 30 inches, the word fair is 

 marked on one fide, and frojl on the other; at 295 is 

 marked the term changeable both for fummer and winter ; at 

 29 are infcribcd on the one fide rahi, and on theother_///oty ; 

 and at 2%\ inches are the words much rain on one fide, and 

 mir.h fn'itx) on the other. E?.ch of thefe larger divifions is 

 ufually fubdivided into ten parts, and by means of a fir.aU 

 Aiding index adapted to the inftrument, the afcent or defcent 

 of the mercuiy may be afcertained for any number of divi- 

 fioiis. Each of thefe tenths is again fometimes divided into 

 ten more, or hundredths of an inch, by means of a Hiding 

 piece of brafs, with a fcale called Nonius and Vf.rniee ; 

 for the ufe of which fee thefe terms, and the ftquel of this 

 article. 



As the common barometer is the bell, and moft to be de- 

 pended upon in accurate obfervations, it may be proper to add 

 fome directions for preparing it : they are collefted chiefly 

 from the publications of Mulchtnbroek, Dcfaguliers, and De 

 Luc on this fubjecl. It appears from many experiments, that 

 the mercury fl;ands higher in tubes of a larger, than in thofe 

 of a narrower bore ; and therefore when obfervations are 

 made with different barometers, fome regard (hould be paid 

 to the difference of their diameters, and it would be defirable 

 to have them conllrutlcd of tubes of the fame diameter. 

 The bore of the tube fliould be large, in order to prevent 

 the effetls of the attradlion of cohefion ; not Icfs than one 

 fourth of an inch ; but if they are one third of an inch dia- 

 meter, they lire better. If a cillern be ufed as a refervoir 

 for the Ilagnant mercur)-, it fhould be large in proportion 

 to the diameter of the tube, at lead ten times greater; that 

 the addition or fubtradlion of the mercury, contained be- 

 tween the greatefl and leaft altitudes, may not fenfibly affecl 

 its depth ; lor the numbers marked on the fcale annexed to 

 the tube, (hew their diltance from a fixed point, and cannot 

 truly indicate the height of the column above the mercury 

 in the ciftern, unlefs its furface coincide with this point, 

 and be immoveable. In order more effectually to preferve 

 the lower furface at the fame height from divifions on the 

 fcale affixed to the inffrument, the father of the late Mr. 

 George Adams firft applied to the barometer a floating gage, 

 by means of which the fame fcrew that renders the barome- 

 ter portable, regulates the furface of the mercuiy in the 

 ciftern, fo that it is always at the place from whence the di- 

 vifions on t!ie fcale com.mence. See Portable Barometer, 

 The tube fliould be preferved free from dufl; till it is 

 ufed ; and for this purpofe it may be hermetically fealed at 

 both ends, and one end may be opened with a file, when 

 it is filled. If this precaution has not been obferved, the 

 infide Ihould be well cleanfed, by waihing it with dcohol 

 highly reilified, and rubbing it with a httle piflon of fliam- 

 my leather fallened to a wire. The mercury (hould be pure ; 

 and may be purged of its air, by previoufly boihng it in a 

 glazed earthen pipkin covered clofe ; and when the tube has 

 been uniformly heated and rendered eleftrical by rubbing it, . 

 the hot mercury fliould be poured into it in a regular cur- 

 rent, through a glafs funnel with a long capUlary tube, fo 

 that the air may not have room to pafs bet.veen the parts of 

 the quickfilver. M. De Luc dircfts, as Mr. Orme had prac 

 tifed many years ago in the conllruclion of his improved di- 

 agonal barometers, that the meicuiy (hould be boiled in the 



tube, 



