suitable receptacle in which to carry such an instrument. The moisture from the body sooner or later affects the metal mechanism; moreover, the knocking and banging which it is likely to receive if carried in an overcoat pocket are equally hurtful. In traveling, it is carried most safely in a grip sack.
A very good place for the pocket aneroid is the observer’s desk and it is advisable to set the zero mark of the altitude scale daily at the index position. Thereby is inculcated a habit of watching the barometric changes far more closely than is the custom when one must go across the room to set the mercurial barometer for a reading. The close observer gets a much better view-point of daily variations than does the casual observer.
Specific Uses of the Aneroid Barometer.—A custom that is wellnigh universal makes the mercurial barometer the standard instrument for the measurement of pressure at Weather Bureau stations. There is no doubt of the wisdom of this practise. For a substantial instrument not easily getting out of order, and susceptible of close reading, no other form of barometer approaches it. Such instruments as the Marvin normal barograph represent the highest skill in precision instruments.
The modern aneroid is quite as essential as the mercury barometer in the equipment of a Weather Bureau station, a maritime observatory, or a meteorological laboratory. For use on vessels it has many advantages over the mercurial barometer, Extras may be carried on various parts of the ship where convenience suggests. Not the least virtue of the ship’s aneroid is the fact that it responds to pressure changes more quickly than does the mercurial barometer.
Aneroid Recording Barometers.—Recording aneroids are sold under various copyrighted names; in Weather Bureau cant such an instrument is known as a barograph. The essential feature is one or more vacuum chambers, a drum moved by a clock, and a ruled sheet of paper on which the record is made. The better type of barograph has a battery of eight vacuum chambers, one upon the top of another. This arrangement permits the movement due to pressure to be multiplied eight-fold. A finer adjustment and a more accurate record is gained thereby. Temperature compensation is effected by the admission of a measured quantity of dry air into one of the chambers.
