The mountain barometer, used for deter mining the altitude of mountains, is the most scientific form which the instrument assumes. All the resources of the instrument maker are here brought into requisition; and many of our distinguished philosophers have devised forms of the instrument known by their names. Among the more recent changes in this instrument are two for which Mr. Bursill obtained patents in 1841, intended to obviate certain slight inaccuracies in the ordinary forms. Another kind of mountain barometer was patented by Mr. Readman, in 1842. Mr. David Napier, in 1848, patented a baro meter of such ingenious construction, that it can not only mark the entire course of atmo spheric pressure for a continuous period of 24 hours, but can register its own observations on a piece of paper.
Sir John Robinson has contrived a simple substitute for the complex mountain barometer. It consists of a wooden box, containing a thermometer and a number of tubes, of a bore somewhat wider than those of self-registering thermometers, open at one end, and blown into bulbs at the other; there is also a small vessel of mercury. The observer notes the thermometer at each station on the mountain and immerses the end of one of the tubes in the mercury. When the observer descends to a lower level, mercury will be found to have entered each tube, to a greater height accord ing to the height of the station to which each tube refers. By exposing each tube to an
air-pump, exhausted till the mercury stands at the same height as at the station, the at mospheric pressure at that station becomes determined.
The making of barometer and thermometer tubes is among the curiosities of the glass manufacture. Few manipulative processes are more striking. The workman collects a quantity of melted glass on the end of a tube, rolls it into a cylindrical form, blows through the tuhe to hollow the glass within, and holds it up to enable another man to attach a rod to the other end of the glass ; the two men then walk backwards in opposite directions, to a distance of forty or fifty feet, elongating the glass as they go, or rather allowing it to elon gate itself; until that which was a short thick cylinder, becomes a long thin tube, with an equable bore throughout its whole length. The tube so made sinks gently to the floor of the glass-house ; and when cold it is cut or broken into convenient lengths.
The filling of the tubes with mercury, and the adjustment of the scales, are delicate processes in the manufacture of a good baro meter.
A remarkable modern form of this instru ment has been already described. [ANsaorn BAROMETER.]