A barometer of the most improved• construction is represented in fig. 19 ; a portion of the tube is shown in fig. 20 ; and a section of its cistern in fig. 21. By help of a screw pressing against. the bottom of a leather bag, inclosed within a cylindrical ivory box, the mercury is always brought up through .a tubular aperture to the same precise level ; or till its convex surface appears to touch a very thin line of light, which is admitted through a slip of ivory applied against the side of the chink or separation of this tube from a wider one immediately over it. The lower end of the mercurial column being thus ad justed; the length is easily measured by drawing gradually down a hollow brass tube, divided at in tervals by wide slits, covered on one aide by thin bits of ivory, till by that softened light a contact is ob served with the edge of a slit and the convex top of the column. Thefine Vernier which the moveable tube carries gives the altitude of the mercury in thousandth parts of an inch. A thermometer is ake wise constantly attached to the instrument, for the purpose of indicating the temperature of the mer., cury, which, from the heat of the band. is carrying, or the influence of the solar beams, is commonly warmer than the external air.
, This mountain barometer is suspended for observa tion by jimbols from a tripod, as exhibited in fig. 18 ; but its several parts can be folded up together into a convenient compass. tolerably well fitted for car riage, as represented in fig. 19. The whole apparatus may not exceed the weight of ten pounds, yet even this, moderate as it might seem, would be felt a seri ous encumbrance by a traveller who is engaged, day after day, in the labour of climbing mountains. The risk which the instrument incurs, besides, in trans porting it perhaps over rough precipices, imposes a perpetual constraint, while, to make correct obser vations with it, must always require time and pa tient attention. A lighter and more compact, though less accurate, barometer will _generally be preferred by the geological traveller, whose object is rather to extend our acquaintance with the altitudes of moun tains, than to aim at a superfluous and often illusory precision. The portable instrument, invented by Sir Henry Englefield, and represented in fig. 14, will, on the whole, answer those views. Its cistern is formed of boxwood, sufficiently tight to hold the mercury, without preventing the access and impression of the external air. When this barometer is inverted, the mercury, therefore, subsides very slowly in the tube, which must be firmly suspended in a vertical posi tion. For greater security, the mercury is now put into a leathern bag introduced within the cistern.
A very simple and convenient sort of portable baro meter was lately invented in France by that celebrat ed chemical philosopher M. Gay-Lussac. (See fig. 15
and 16.) It consists of rather a wide syphon tube, filled with mercury, and sealed hermetically at the inverted end, having a very fine capillary hole formed about an inch under this, by nicely directing the flame of a blow-pipe against the aide of the glass, and drawing a melted spot of it out to a point. The lower portion of the principal branch has its bore contracted to less than the tenth part of an inch, to prevent the mercurial column from dividing in the act of inverting it. The mercury is boiled as usual, and the tube may be con cealed in a walking stick, or lodged, like the complete mountain barometer, in a cylinder of brass, with move able sliders bearing the divisions of a Vernier at both ends. (See fig. 17 .) For greater simplicity, however, the larger divisions might be engraved on the tube itself. This kind of barometer is of ready use, and very lit tle exposed to hazard in carriage. It is commonly held in a reclined or inverted position ; but, in mak ing an observation, it must be gently turned back, and kept perpendicular till the mercury descends through the contracted bore, and slowly rises again in the opposite short branch ; the scale is noticed at both ends of the incurred column, and the difference of those indications, gives its correct altitude.
A modification of the conical barometer, which, in travelling, we have ourselves employed with great ease and advantage, should likewise be mentioned. The principal part of it consists of a small stop-cock made of steel, and represented in fig. 13. A glass tube of 31 or 32 inches long, with a bore of the tenth part of an inch hermetically sealed at the top, and filled with quicksilver, is cemented into the one end of the ; and into the other end is ce mented an open and wider tube, 16 inches or more in length, and having its diameter above the eighth part of an inch. This compound tube is lodged in a walking-stick, divided into inches and tenths through its whole extent, or only at the upper part, if uniform tubes be selected. In making an observation, the cock is turned, and the instrument inverted. The up per column then descends partly into the lower tube, till it becomes shortened to the proper altitude.
We have already stated the principles on which the calculation of barometrical measurements proceeds. But there still are some points, either assumed or overlooked, which may considerably modify the re sults. It is presumed,that, at equal successive heights, the temperature of the atmosphere decreases uniform ly. This property, however, does not hold strictly, and it may be shown from a comparison of the best observations, that the decrements of heat follow a quicker progression in the higher regions. But we shall soon have another opportunity to examine this subject, and trace out its various consequences.