“Aneroid barometers, if often compared with good mercurial columns, are similar in their indications, and valuable; but it must be remembered that they are not independent instruments, that they are set originally by a barometer, require adjustment occasionally, and may deteriorate in time, though slowly.”
“The aneroid is quick in showing the variation of atmospheric pressure; and to the navigator who knows the difficulty, at times, of using barometers, this instrument is a great boon, for it can be placed anywhere, quite out of harm’s way, and is not affected by the ship’s motion, although faithfully giving indication of increased or diminished pressure of air. In ascending or descending elevations, the hand of the aneroid may be seen to move (like the hand of a watch), showing the height above the level of the sea, or the difference of level between places of comparison.”
In the admiral’s Notes on Meteorology, he says, “The aneroid is an excellent weather glass, if well made. Compensation for heat or cold has lately been introduced by efficient mechanism. In its improved condition, when the cost may be about £5, it is fit for measuring heights as far as 5,000 feet with approximate accuracy; but even at the price of £3, as a weather-glass only, it is exceedingly valuable, because it can be carried anywhere; and if now and then compared with a good barometer, it may be relied on sufficiently. I have had one in constant use for ten years, and it appears to be as good now as at first. For a ship of war (considering concussion by the fire of guns), for boats, or to put in a drawer, or on a table, I believe there is nothing better than it for use as a common weather-glass.”
Colonel Sir H. James, R.E., in his Instructions for taking Meteorological Observations, says of the aneroid, “This is a most valuable instrument; it is extremely portable. I have had one in use for upwards of ten years, and find it to be the best form of barometer, as a “weather-glass,” that has been made.”
One of the objects of Mr. Glaisher’s experiments in balloons was “to compare the readings of an aneroid barometer with those of a mercurial barometer up to five miles.” In the comparisons the readings of the mercurial barometer were corrected for index-error and temperature. The aneroid readings, says Mr. Glaisher, “prove all the observations made in the several ascents may be safely depended upon, and also that an aneroid barometer can be made to read correctly to pressures below twelve inches.” As one of the general conclusions derived from his experiments he states, “that an aneroid barometer read correctly to the first place, and probably to the second place of decimals, to a pressure as low as seven inches.” The two aneroids used by Mr. Glaisher were by Messrs. Negretti and Zambra.
Aneroids are now manufactured almost perfectly compensated for temperature. Such an instrument therefore ought to show the same pressure in the external air at a temperature say of 40°, as it would in a room where the temperature at the same time may be 60°; provided there is no difference of elevation. To test it thoroughly would require an examination and a comparison with barometer readings reduced to 32° F., conducted through a long range of temperature and under artificially reduced pressure. A practical method appears to be to compare the aneroid daily, or more often, for a few weeks with the readings of a mercurial barometer reduced to 32°; and if the error so found be constant, the object of the compensation may be assumed to be attained, particularly if the temperature during the period has varied greatly.
Directions for using the Aneroid.—Aneroids are generally suspended with the dial vertical; but if they be placed with the dial horizontal, the indications differ a few hundredths of an inch in the two positions. Hence, if their indications are registered, they should be kept in the same position.
The aneroid will not answer for exact scientific purposes, as it cannot be relied upon for a length of time. Its error of indication changes slowly, and hence the necessity of its being set from time to time with the reading of a good barometer. To allow of this being done, at the back of the outer case is the head of a screw in connection with the spring attached to the vacuum box. By applying a small turnscrew to this screw, the spring of the vacuum box may be tightened or relaxed, and the index made to move correspondingly to the right or left on the dial. By this means, besides being enabled to correct the aneroid at any time, “if the measure of a height rather greater than the aneroid will commonly show be required, it may be re-set thus: When at the upper station (within its range), and having noted the reading carefully, touch the screw behind so as to bring back the hand a few inches (if the instrument will admit), then read off and start again. Reverse the operation when descending. This may add some inches of measure approximately.”—FitzRoy.
Fig. 34.
