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fiiould be defended from the rain by a cover of wax-clolli. 

 Ill order to prevent its being unduly affetled by heat, it 

 fliould be kept at a diftance from the body of the man 

 •who carries it, and be protcfted from the fun by an um- 

 brella, when it io near the place of obfervation. To the 

 apparatus a plummet fhould be annexed, in order to af- 

 ccrtain its vertical pofition, and a three-legged frame or 

 tripod will ferve to keep it firm in that pofition at the 

 time of obfervation. The fcale of this barometer is an- 

 nexed to the long tube ; it commences at a point on a 

 level with the upper end of the fhort one, and riles in 

 the natural order of the numbers to 21 inches. Below 

 the above point, the fcale is transferred to the (liort tube ; 

 and defcends upon it in the natural progrelTion of the num- 

 bers to 7. The interval of 27 inches, comprehended be- 

 tween the point marked 20 in the upper tube, and that 

 which correfponds to 7 in the lower, is divided into 27 

 parts, which are inches. Thcfe inches are again divided 

 into lines, fourths, fixteenths, and e\'cn thirty-lecond parts 

 of lines. The adhcfion and friftion of the mercury in the 

 tubes will not allow of a more minute fubdivifion. As the 

 inercury falls in the one tube, it will rife in the other ; and 

 therefore the total altitude will be found by adding that 

 part of tl e fcale which the mercury occupies in the long 

 tube, to that p.irt of it which the mercury does not occupy 

 in the fhort one. In ellimating, however, the total fall or 

 rife in the long lubs, every fpace muil be reckoned twice ; 

 becaufe in barometers of this kind, half the real variation 

 only appears in one of the branches. 



One of the thermometers, exhibited in fg. 95, is defigned 

 for afcertainirg the correftions that are to be made in the 

 height of the mercury on account of any variation in the 

 temperature of the air by heat and cold. For this purpofe it 

 is placed near the middle of the longer tube, tliat it may 

 partake as much as pufTible of its mean heat. The ball is 

 nearly of the fame diameter with that of the tube of the ba- 

 rometer, that the dilatations or condenfations of the fluids 

 contained in them may more exactly correfpond ; and this 

 ball fhould alfo be enclofed in wood that it may participate, 

 as well as the barometer, of the heat of the bottom ot the 

 box. The fcale of the thermometer is divided into 96 parts, 

 between the points of boiling water and melting ice. M. 

 de Luc, having found that an iucreafe of heat, iufficient to 

 raife the thermometer through this interval, augmented the 

 height of the mercury in the barometer, when it was at 27 

 French inches, precifely fix lines, was led to divide it into 

 96 equal parts ; fo that one of thefe parts correfponded 

 to -jlg^^' °^ ^ ^'"^ '" ^^"^ height of the barometer : and this 

 quantity therefore mult be added to or fubtraCted from the 

 faid height, for every degree of variation of the thern-iometcr 

 thus graduated. He placed the term o, one eighth part of 

 the above interval above the lower point : fo that there are 

 12 degrees below and 84 above it; becaufe as 27 French 

 inches exprefs the mean height of the barometer, fo the 12th 

 degree above freezing is nearly the mean altitude of the ther- 

 mometer. Hence by taking thefe two points, the one for the 

 mean altitudeandtheother forthe mean heat, there will be fuw- 

 ercorreflions neceffary forreducing allobfervations tothefame 

 ftate, than if any higher or lower points had beeu taken. 

 The divillons above o or -zero, are coiifidered as politive and 

 denoted by -j-, and thofe below as negacive and exprefTedby — . 



If the barometer remains at 27 French inches, and the 

 ih' r nometer at o, according to the above explained gradu- 

 ation, no correftions are neceffary. But if, while the ba- 

 rometer continues at 27 inches, the thermometer Ihould rife 

 any number of degrees above o, fo many fixtetnlhs of a line 

 muft be fubtraited from the 27 inches, in order to obtain 

 the true height of the barometer produced by the weight of 



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the atmofphere, and to reduce this obfervation to the fVate 

 of the common temperature. On the other hand, if the 

 thermometer fhould fall any number of degrees below o, 

 while the barometer remains at 27 inches, fo m-m.-^ fixteenths- 

 mud be added to that height in order to obtain the 

 true altitude. Thefe correftions are very fmiple and 

 eafy when the height of the barometer is at or near 27 inches. 

 But if it fall feveral inches below tins point, as the port- 

 able barometer frequently mull:, according to the ftations in 

 which it is placed for the purpofe of meafuring altitudes, 

 the dilatations will no longer correfpond with the degrees- 

 of heat, after the rate of -^'^th of a line for every degree 

 of the thermometer ; becaufe the columns of mercury being 

 fliorlened, the quantity of fluid to be dilated mull be dinii- 

 nifhed ; and, according to a general ilarement, the quantity 

 of dilatation forthe fame degree of heat will be as much di- 

 minifhed as the column is fhortened. If, then, it fhould be 

 flill found convenient to reckon the dilatations by fixteenths 

 of a line, thcfe fixteenths muft be counted on a fcale, of 

 which the degrees fliould be as much longer than the de- 

 grees of the firll fcale, as the fhortened column of mercury 

 is Icfs than 27 inches, the height to which the length of 

 the degrees of the firll fcale was adapted. E. G. Let 

 the mercury defcend, in confequence of the elevation of the 

 barometer, to 13I inches, or half the mean column, and let 

 the thermometer afcend ten degrees above the mean heat j 

 then according to the rule -|-Jths fliould be dcdufted from 

 the mean column for this temperature ; but ten half-fix- 

 teenths only or xV'^^ mull be fubtrafted from the column 

 of 13I inches, becaufe the fum of its dilatations will be half 

 that of the former ; the quantities of fluid being to one ano- 

 thtr in that proportion. As it would occafion confiderablc 

 embarraflment to fubdivide the fixteenths of correftion iiita 

 fmaller traftions proportional to every half inch of defcent 

 in the barometer, the fame end may be obtained in a much 

 more eafy manner by reckoning the correftions on different 

 fcales ot the fame length, with the degrees longer as the co- 

 lumns of the barometer are fliorttr. E. G. The degrees of 

 correftion on a fcale applicable to the column of 13I inches 

 will be double in length to thofe of the fame degrees adapted, 

 to the column of 27 inches, and confequcntly the number of 

 correftions vi'ill be reduced to likewife one half. M. de 

 Luc conitrudtcd, in the manner which he has minutely de- 

 fcribed ( Recherches, vol. ii. p. 26, &:.), on a piece of vel- 

 lum, fcales with thefe properties for no lefs than 23 columns 

 of mercury, bting all thofe between 28 inches and 29 in- 

 clulive, reckoning trc.ra half inch to half inch, within which 

 extremes every praftical cafe will be comprehended. This, 

 vellum he wrapped on a fmall hollov/ cylinder, including 

 a fpring, hke a fpring curtain, and he fixed it on the right 

 fide of the thermometer. The vellum was made to pafs 

 from right to left, behind the tube of the thermom.eter, and 

 to move along its furface. The obferver, in ellimating the 

 neceffary correftions, draws out the vellum till the fcale cor- 

 refponding to the obferved altitude of the barometer, touchea 

 the thermometer, and he counts them on that fcale. The 

 vellum is then let go, and it is gently furled up by the fcrgw. 

 M. De Luc, having provided the neceffary apparatus for 

 the accurate menfuration of heights, proceeded to eflablifli by 

 experiment the altitudescorrefpondingtothe different defcents 

 of the mercur)' ; and he made choice of Saleve,?. mountain near 

 Geneva, about 3000 Fiench feet high, for the fcene of hia 

 operations. The height of this mountain was twice meafu- 

 red by Icvelhng, and the refult of the mcnfurations, at the 

 interval of fix months, gave a difference of only io| inches- 

 On this mountain he felefted no lefs than 15 different fta- 

 tions, rifing at the rate of nearly 200 feet one above another; 

 and Itere he propofed to make fughanumber of obfervations a& 



-would 



