Fig. 398.—Hypsometer, or boiling point thermometer.
Fig. 399.—Case for hypsometer.
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843.—The modern form of instrument is shown in Fig. 398. The boiler shown immediately over the lamp is filled about half full of rain water by lifting off its covering tube C. The covering tube has a smaller tube, about 3 inches long and ½ inch diameter leading upwards from it, through which the thermometer bulb is passed into the boiler. This tube is covered by the jacket J, formed of four telescopic tubes that are extended, as shown in the figure, for use, but which close up quite compactly when the instrument is put in its case. The upper drawer of the jacket tube is about ¾ inch diameter, so that the tube enclosing it passes over the leading tube when the apparatus is closed. The lamp, which is filled with pure spirit, draws out from the bottom of the outer casing O. It carries a wick holder with screw cap, and this again has a covering cap to secure the spirit perfectly when the instrument is carried about. The inner casing A is perforated with holes to admit air at the level of the body of the lamp. When the lamp is lighted and complete for use it is placed vertically in its outer case O, which is jointed in two parts and perforated by large holes surrounding it top and bottom: the bottom holes are covered with wire gauze. By this arrangement the flame is not seriously disturbed by wind or rain.
844.—The Thermometer, upon which the action of the instrument depends, has a stout stem about 6 inches long and ¼ inch diameter, with a very fine, flat, oval bore about ·01 inch wide and not much over ·005 inch in thickness. The stem is divided very openly for about 25° below 100° centigrade, each degree being subdivided into 10, below 212° if Fahrenheit scale be used, with each degree divided into 5. The divisions are filled in with lamp-black, and the stem is backed with white enamel to give clear reading. The thermometer T when in use is surrounded by a vulcanized indiarubber collar I which slips over its stem to adjust it to position in the boiler tube as shown.
In placing the thermometer in its jacket, it is important to hold it erect to be sure it passes into the leading tube from the boiler, as there is generally just room for it to catch by the side of this tube, where if it were pressed down it would break the bulb. When the thermometer is out of use the rubber collar is removed, and the thermometer is placed in a tubular metal case which is lined with indiarubber tubing, so that no jar can injure it.
The whole apparatus when closed is carried in a solid leather case, which contains divisions for the separate parts of the apparatus, and a strap for passing over the shoulder for carrying it. Fig. 399 shows the general form of case.
845.—Use of the Hypsometer.—Saussure calculated, from data of his ascents of Swiss mountains, that the temperature of boiling water decreased 1° centigrade for every 978·5 feet of ascent, where the mean temperature of the atmosphere was estimated at 0° centigrade, or freezing point. If the temperature of the surrounding atmosphere be taken as 5·5° centigrade, the ascent per degree of that scale is 1000 feet. This becomes, therefore, the most convenient data to calculate from, allowing 3·9 feet per 1000 per degree centigrade for temperature above or below 5·5° centigrade at any two stations of observation, of which the difference of level is required. Thus:—If at the first station the temperature of air be 15·6 centigrade, the boiling point 95·5° centigrade; second station temperature of air 14·1° centigrade, boiling point 94·2° centigrade, the barometrical pressure of the lower station being taken as a constant, or referred to the aneroid for correction; then 15·6° - 5·5° = (9·1) (3·9) = 29·2 + dif. 95·5 - 94·2 = (1·3) (1000) = 1329·2 - dif. external temperature (15·6 - 14·1) (3·9°) = 1323·4 difference of level in feet.
Sometimes the thermometer is divided to Fahrenheit degrees, subdivided into 5 to read by interspace and line to ·1° F. This may be changed to centigrade for use of the above formula by taking 32° F. lower than the reading and multiplying by 5/9. Thus—