BAR 



the furface of tlie mercury at X may be raifed an incTi 

 higher (meafuHiig fiom its furface at P) than btfcre ; an J 

 tliat the breadth of the cavity of the tiilje at X, and of the 

 bafon at /", are fuch, that by this afcent of tlie mercury 

 tlierc may be a cubic inch of it in the cavity A' more than 

 before, and confequently in the bafon a cubic inch lefs. 

 Now, upon this fiippofition, there will be a cubic inch of 

 water in the bafon more than there was before, becaufe the 

 water will fucceed the mercury to fill up its place. Upon 

 this account the whole machine will be rendered heavier than 

 before by the weight of a cubic inch of water ; and there- 

 fore will link, according to the laws of hydroftatics, till a 

 cubic inch of that part of the rod W S, which was above 

 the furface of the water at IV, comes under it. Then if we 

 fuppofe this rod fo fmall that a ctibic inch of it fliaU be 14 

 inches in length, the whole machine will fink 14 inches 

 lower into the fluid than before; and, confequently, the 

 furface of the mercury in the baion will be pref'i.d more 

 than it was before, by a column of water 14 ineh^-s high. 

 But the prefTure of 14 inches of water is equivalent to one 

 of mercur)' ; tliis additional prefTure will make the n ercury 

 afcend at X as much as the fuppofed variation in the v.-eight 

 of the air did at firft. This afcent will give room for a fe- 

 cond cubic inch of water to enter the baloii ; the niacltine 

 will therefore be again rendered fo much heavier, and will 

 fubfide 14 inches farther, and fo on ;';; ttifinitum. If the rod 

 was fo fmall that more than 14 inches of it were required 

 to make a cubic inch, the variation of this machine would 

 be negative with refpeft to the common barometer, and in- 

 flead of coming nearer to an equilibrium with the air by its 

 afcent or defcent, it would continually recede farther from 

 it : but if lefs than 14 inches of rod were required to make 

 a cubic inch, the fcale of variation would be finite, and 

 might be made in any proportion to the common one. 



The fame author has alfo contrived a compound barometer, 

 in which the fcale of variation fliall bear any proportion to 

 that of the common one. /I B C (Jig. 90. ) is a compound 

 tube hermetically fcalcd at A, and open at C ; empty from 

 ji to D, filled with mercury from thence to B, and from 

 thence to E with water. It appears from the nature of a 

 fiphon, that if H, B, G, be in the fame horizontal hue, the 

 column of inercury D H will be in equilibrio with the co- 

 lumn of water G E, and a column of air of the fame bafe, 

 and will therefore vary with the fum of the variations of 

 thefe. He has fub'ioined a calculation, whence it appears, 

 that if the tubes AFunA FC are of an equal bore, the vari- 

 ation in this is lefs than that of the common barometer, in 

 the proportion of 7 to 13 ; but if the diameter of ^7^ be 

 to that of FC as 5 to I, the variations will be to thofe in the 

 common barometer, as 175 to i ; but if the proportion of 

 the diameters be greater, the variations will be infinite in 

 refpeft to thofe of the common barometer. Of the prac- 

 tical utility of this conftruAion the author had no experi- 

 ence. Rowning's Nat. Phil, part ii. diff. 4. 



Another contrivance for enlarging the fcale of the baro- 

 meter is exhibited '^n fig. 91. uiB is the tube of a common 

 taiometer, open at B, and fcaled at yl, fufpendcd at the 

 end of a lever which moves on the fulcrum E, Z) i' a glafs 

 tube fixed, and ferving for a ciilern, which is wide iioiigh 

 to admit the free motion of the barometrical tube j4B. 

 AB, when filled with mercury, is nearly counterbalanced by 

 the long end of the lever. When the atmofphere becomes 

 lighter, the mercury defcends in the long tube, and the fur- 

 face of the mercury rifing in the ciflcrn, pufhes up the tube 

 jiB, which caufes the lever to preponderate, and to point 

 out by its index m.oving along a circular arc, the moft mi- 

 nute variations. This apparatus, however, is fubjefl to the 



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inconvenience of the friftion as well as weight of the lever, 

 when put in motion by the rife or fall of the tubcy^fi. 



Whillt: fome have endeavoured to enlarge the variations of 

 the barometer, ethers have endeavoured to make it more 

 convenient, by reducing the length of the tube. M. Amou- 

 tons, in 168S, firH propofcd this alteration in the flrufture 

 of barometers, by joining fcveral tubes to one anoth.er, al- 

 ternately filled with mercury and with air, or fome other 

 fluid ; a;id the number of thefe tubes may be incVeafed at 

 pleafurc : but the contrivance is more ingenious than 

 ufeful. 



M. Mairan's reduced barometer, which is only three 

 inches long, ferves the purpofe of a manometer in difcovering 

 the dilatations of the air in the receiver of an air-j.ump ; 

 and infirume'its of this kind are now generally applied to 

 this ufe. Ss-C K\^-Pump, and Gage. 



For an account of a felf-regiltering barometer by the 

 Rev. Arthur M'Gwirc, fee Irifh Tranfaclions, vol. iv. p. 141. 



The barometer lately invented by Alexander Keith, Efq. 

 F. R. S. and F. A. S. Edinb. marks the rife and fall of the 

 mercury from two diiTerent times of obfervation. This in- 

 ftrumcnt confifts of a glafs tube ABCD (jig. 92.) bent in 

 the manner rcpreftntcd in the figure, open at D, and her. 

 metically lealed at A. The length from ^ to i^ is 8 inches, 

 and its calibre about \ of an inch ; from i? to C it is 31 J 

 inches long, and about \ inch calibre; and from C Xa D \\ 

 long, and 5 inch calibre. The tube is filled with mercur}', 

 the length from A to E being 29^ inches. When the tube 

 is hung perpendicularly, the mercury will fall from B towards 

 E, leaving a vacuum from A to B. When the atmofphere 

 becomes heavier, the mercury falls in the tube D C ; and 

 when lighter, it rifes. The range of the fcale is about 3 

 inches, being equal to that of a common barometer of the 

 bell conftruftion, which has a bafon with a very broad fur- 

 face. This inftrument moves in a direftion contrary to that 

 of the common barometer, the one rifing while the other 

 falls. The tube Z) C is reprefcntcd on a larger fcale in 

 jig. 93. ; F is the float, having the float-wire fixed to it, ter- 

 minating in a knee at a right angle between the indexes hL, 

 where it embraces a very fmall wire flretched along the fcale, 

 and thereby raifes or lowers them as the mercuiy rifes or 

 falls in the tube D C. The barometer is prepared for ob- 

 fervation, by bringing down the one and raifmg the other 

 index, till both touch the knee of the float-wire. When 

 next obferved, the upper index will point out the greatcft 

 depreffion of the mercury, or lightneis of the atmofphere ; 

 and the lower the greateft rife of the mercury, or weight of 

 the atmofphere fince the fcale was prepared. By thefe 

 means, the variations of the atmofphere are more truly 

 pointed out than by the common barometer; for it often 

 happens that during tempeftuous weather, or before it, the 

 mercuiy both rifes and falls within a few hours, or during 

 the night time ; which variations cannot be noticed by any 

 of the barometers now in ufe. The fudden fall and rife, or 

 even the rife and fall of the mercury, always denote an ex- 

 traordinary agitation in the atmofphere. In a common ba- 

 rometer the mercury may be at the fame height in the 

 morning that it was the night before ; which leads to a 

 conclufion that as there has been no agitation of the mer- 

 cury, there will be calm or fettled weather : but this new 

 barometer will often (hew in fuch cafes, that the one float 

 has been railed -,%, and the other deprefled as much ; wliich 

 inftead of indicating ealm weather denotes that tempeftuous 

 weather may be expefted. 



The weight of the atmofphere at great heights might be 

 difcovcred by fufpending this inftrument to an air-balloon. 

 Edinb. Tranf. vol. iv, 1798. 



The 



