Surveying and Drawing Instruments by C.F. Casella & Co. Ltd

Author:C.F. Casella & Co., Ltd.
Language: eng
Format: epub, pdf
Published: 1911-03-25T05:00:00+00:00

No. 281.

There is, however, a second cause of variation, which (though included in the recognized formulae for reducing barometrical deter minations of altitude) has not been included in any altitude scale yet published. This cause is the variable temperature of the air,* which will evidently affect the result by affecting the density of the air, and therefore the ratio of pressure to altitude. In the engineering aneroid invented by Mr. Rogers Field, B.A., Assoc. Inst. C.E., this variation is for the first time taken into account by the scale being made adjustable for temperature. The principle of adjustment is that of shifting the altitude scale according to the temperature of the air. There is nothing novel in movable scales of themselves, as they are frequently used with aneroids for the purpose of shifting the zero of

* This must not be confounded with the changes produced by alterations in the temperature of the instrument itself, which may be neglected, as the compensation for temperature is as nearly perfect as possible.

70 C. F. CAS ELLA & CO., LTD.,

ANEROID BAROMETERS FOR SURVEYING AND MINING PURPOSES —Continued.

the scale to correspond with the varying position of the hand. This method of using them is, however, radically wrong, as the shifting of the scale makes it inaccurate, and the novelty consists in taking advantage of this inaccuracy to obtain a valuable means of adjustment. The scale that is inaccurate for one temperature is practically accurate for another, so that we have only to shift the scale into certain different fixed positions to obtain a series of different scales suitable for different temperatures of the air.

The altitude scale adopted in this aneroid is that of the Astronomer Royal (which has been compared with the formulae of Laplace, Guyot, Baily, Plantamour and other authorities, and found to give results lying between them), and as the instrument is intended for the accurate measurement of moderate altitudes, the range is purposely limited so as to give an open graduation. The adjustment for the temperature of the air is applied by shifting the scale in accordance with the figures engraved on the outside of the instrument, marked A A on wood-cut. The rim which holds the glass should be slightly raised, so as to be free from the locking-pin, and then turned until the figures corresponding to the air temperature are opposite to the pin, when the glass should be depressed so as to re-lock it.

The process of observation is extremely simple. The first thing is to determine, either by observation or estimation, the air temperature likely to prevail during the series of observations ; if this is done within 5° F. it will be sufficiently accurate (within about i per cent.). The scale must then be set to this temperature in the manner above explained. Subsequently the readings must be taken from the outer scale of feet, and the difference will give the difference of elevation. The following example of actual observations taken between Hampstead and London will explain the proceeding :—

Temperature of air 40° F.

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