BARO M E T E R. 



289 



Plate LIU. 





Barometer, is hermetically sealed at A, ought to be about S3 

 inches in length, and 1\ or 3 lines in diameter of 

 bore. Since tli'e height cf the mercury in the tube 

 must be reckoned from the surface of die mercury 

 in the bason, the width of the latter ought to be 

 such, that the elevation or depression of the mercury 

 in the tube may have little effect in changing the 

 level of the mercury in the bason. In the more im- 

 proved barometers, contrivances, which we shall af- 

 terwards explain, have been adopted for preserving 

 the lower surface of the mercury at the same level ; 

 but this is obtained sufficiently well for common pur- 

 poses, by giving the bason a considerable diameter 

 with respect to that of the tube. 

 Common The common barometer, represented in Fig. 3, 



barometer, differs but little from the Torricellian tube. Instead 

 F 'g- 3 - of a bason, a small reservoir is usually attached to the 



lower extremity of the tube, or rather the tube itself 

 k swelled out into a bulby form, as represented at 

 Fig. 4. Fig. 4. In this form, however, the instrument is 



by no means accurate, particularly when the diame- 

 ter of the bulb, as is usually the case, differs but lit- 

 tle from the diameter of the bore of the tube. In 

 order to keep the surface of the mercury in the ba- 

 son always at the same level, the father of the late 

 Mr George Adams constructed the bason wholly, or 

 in part, of leather, and by means of a screw at the 

 bottom, adjusted the surface of the mercury in it, so 

 as to have it always at the place from which die di- 

 visions on the scale commence. 



In this country, the lowest station of the mercury 

 is observed to be about 28 inches, and its highest 

 31 inches above the level of the mercury in the bason : 

 and when the instrument is to remain in a fixed posi- 

 tion near the surface of the earth, we may consider 

 the interval between these two points, as the range 

 >f die barometrical scale. The scale which, conse- 

 quently, will embrace three inches, may be subdivided 

 iEto smaller divisions, according to the degree of 

 nicety required. Each inch is commonly divided in- 

 to ten equal parts; and these are subdivided into 

 hundredths of an inch, by a contrivance called a ver- 

 nier scale. ( See Vernier. ) By this means the height 

 of the mercury is ascertained, by inspection, to the 

 -4-jth of an inch. For nice purposes, the vernier 

 may be made to indicate the -r^yth of an inch. 



Besides these lineal divisions, the scale is divided 

 into other points, having a reference to the instrument 

 in its capacity of indicating the probable state of the 

 veather. At 31 inches, the highest point of the scale 

 of variation, set fair ia marked on the one side, and set 

 frost on the other. At 30 inches, fair is written, in 

 like manner, on the one side, and frost on the other ; 

 and at half an inch below is written the word change- 

 able, which answers both for summer and winter. 

 Portable The common barometer answers sufficiently well 



barometers. f or m03 t purposes, when the observations are made 

 at the 6ame place ; but as many of these necessarily 

 imply a change of situation, it soon became an object 

 of importance to construct barometers in such a 

 manner that they might be conveyed from one place 

 to another without much inconvenience or risk. 

 Barometers of this kind, which are called portable 

 'meters, are chiefly employed for measuring 

 VOL. III. 1'AJtT II. 



heights. They have assumed, under the hands of dif- BSu-wnater. 

 ferent artists, a variety of forms. '."""** 



Derham mentions a portable barometer ; but as 

 many circumstances connected with the accuracy of 

 the instrument, are entirely overlooked in its con- , 



struction, it does not merit a particular description. 

 [Phil. Tram. vol. xx. N 236.) The portable ba- 

 rometer consists, in general, ot a tube of the usual 

 length, passing through the upper part of a wooden 

 cistern, to which it is glued, and the bottom of which 

 is made of leather. The tube being filled with mer- 

 cury, which has been previously well purged of air, 

 and placed in a proper position, the superfluous mer- 

 cury descends into the cistern, and assumes a level 

 in the tube, corresponding to the weight of the ex- 

 ternal air. The surface of the mercury in the cistern 

 is adjusted to the same level by a screw, which presses 

 more or les3 against the flexible leather at the bot- 

 tom, and raises or depresses it at pleasure. From 

 the line of this level, which is called zero, the scale 

 commences, and is reckoned upwards to the height of 

 about 32 inches: the actual divisions of the sc,ale begin 

 at ab >ut 15 inches. Various methods have been em- 

 ployed for constructing the portable barometer in a 

 manner best suited to carriage, to placing the tube 

 in a vertical position at the time of observation, to 

 ascertaining the surface of the mercury, and to ma- 

 king allowances for the expansion and contraction 

 of the mercury by changes of temperature : but it 

 will be sufficient to describe the instrument hi its pre- 

 sent most improved form. 



Figures 1, 2, and 3, represent the portable barome- Trough- 

 ter as constructed by Mr Troughton, and first made ton's por- 

 by him in 1785. The greatest peculiarity in this lab,e karo- 

 instrument, according to the opinion of this ingenious p| et t er T IV 

 and philosophical artist, consists in the excellent |.; e j <> ] 

 manner in which the mercury in the cistern is set to 

 the zero of the scale of inches. For this purpose, a 

 glass cylinder of about 2.5 inches diameter, and as 

 much in length, contains the mercury. An external 

 covering of hollow brass, terminating in a female 

 screw a little above and below the glass, admits male 

 screw pieces, whose ends, well leathered, being pres- 

 sed hard against the ends of the glass, prevent the 

 escape of the fluid. Near the upper end of the brass 

 cover are two slits made horizontally, one before, 

 and the other behind, exactly similar and opposite to 

 each other. At bottom is a screw, seen better in 

 the section Fig. 2. which, acting upon the usual leather Fig. &. 

 bag, forces the quicksilver upwards at pleasure, and, 

 by filling every part, renders the instrument portable. 

 But the primary design of this screw is, to furnish 

 the means of adjusting the surface of the mercury in 

 the glass cistern, so as just to shut out the light from 

 passing between it and the upper edges of the slits 

 in the brass cover. This is the mode of adjusting to 

 zero ; and it follows, that the upper edges of the slits 

 must represent the beginning of the scale of inches. 

 The frame is entirely made of a brass tube, and above 

 the cistern is of about 1.1 inch diameter. The first 

 ten inches of the lower end is occupied by a thermo- 

 meter, whose bulb, bent inwards, is concealed within 

 the frame. At about three inches higher, it attaches 

 to the stand by a ring, in which the frame turns 

 2o 



