=93. Mountain Thermometer; sometimes called Hypsometric Apparatus.=--We have now to examine the construction of the boiling-point thermometer, and its necessary appendages, as adapted for the determination of heights. Messrs. Negretti and Zambra's arrangement of the instrument is shown in figures 72 and 73. [Illustration: Fig. 72.] [Illustration: Fig. 73.] The thermometer is made with an elongated bulb, so as to be as sensitive as possible. The scale, about a foot long, is graduated on the stem, and ranges from 180 deg. to 214 deg., each degree being sufficiently large to show the divisions of tenths of a degree. A sliding metallic vernier might perhaps with advantage be attached to the stem, which would enable the observer to mark hundredths of a degree; which, however, he can pretty well do by estimation. The boiler is so contrived as to allow, not only the bulb, but the stem also of the thermometer, to be surrounded by the steam. The arrangement is readily understood by reference to the accompanying diagram, fig. 73. _C_, is a copper boiler, supported by a tripod stand so as to allow a spirit-lamp, _A_, made of metal to be placed underneath. The flame from the lamp may be surrounded by a fine wire gauze, _B_, which will prevent it being extinguished when experimenting in the external air. _E E E_, is a three-drawn telescope tube, proceeding from the boiler, and open also at top. Another tube, similarly constructed, envelops this, as shown by _D D D_. This tube is screwed to the top of the boiler, and has two openings, one at the top to admit the thermometer, the other low down, _G_, to give vent to the steam. As the steam is generated, it rises in the inner tube, passes down between the tubes, and flows away at _G_. The thermometer is passed down, supported by an india-rubber washer, fitting steam tight, so as to leave the top of the mercury, when the boiling-point is attained, sufficiently visible to make the observation. The telescopic movement, and the mode of supporting the thermometer, enable the observer always to keep the bulb near the water, and the double tube gives all the protection required to obtain a steady boiling-point. Some boiling-point thermometers are constructed with their scales altogether exposed to the air, which may be very cold, and consequently may contract to some extent the thread of mercury outside the boiler. The steam, having the same temperature as the boiling water, keeps the tube, throughout nearly its whole length, at the same degree of heat, in the apparatus described. The whole can be packed in a tin case very compactly and securely for travelling, as in fig. 72.