Metallic barometers (in outer shape and size like aneroids) have not yet been tested adequately in very moist, hot, or cold air for a sufficient time. They, as well as sympiesometers, are likewise dependent or secondary instruments, and liable to deterioration. For limited employment, when sufficiently compared, they may be very useful, especially in a few cases of electrical changes not foretold or shown by mercury.
The Sympiesometer is considered to be more sensitive than the marine barometer, falling sooner, and rising earlier: but this is partly in consequence of the marine barometer tube being contracted, to prevent oscillation or "pumping." In the sympiesometer a gas is used, which presses on the confined surface of the liquid with an uniform pressure at an equal state of temperature. The liquid is raised or depressed by an increase or diminution in the density of the atmosphere, and change of temperature is allowed for, by the sliding scale of the instrument being always set to agree with the height of the mercury in the attached thermometer, bringing the pointer on the sliding scale of the sympiesometer to the same degree on the inverted scale (over which it slides) as is indicated by the thermometer. The height of the fluid, as then shown by the sliding scale, indicates the pressure of the atmosphere.
As the instrument is delicate, great care should be taken, in carrying or handling, to keep the top always upwards, and to exclude casual rays of the sun, or a fire, or lamp.
Oil sympiesometers seem to be affected more than mercurial, or others, and much more than the barometer, by lightning or electricity. That they, and the hermetically sealed "Storm Glasses," are influenced by causes besides pressure and temperature, appears now to be certain.
The daily movement of the barometer may be noted (in a form or table of double entry) at the time of each observation, by a dot at the place corresponding to its altitude, and the time of observing; which dot should be connected with the previous one by a line. The resulting free curve (or zig-zag) will show at a glance what have been the movements during the days immediately previous, by which, and not merely by the last observation, a judgment may be formed of the weather to be expected.
Such a diagram may be filled up by uncorrected observations, its object being to serve as a weather guide for immediate use, rather than for future investigation. If closely kept up, it will prove to be of utility, and will in some degree reward the trouble of keeping a regular record. For purely scientific objects much more nicety and detail are required.
Hesitation is sometimes felt by young seamen, at first using the vernier of a barometer, for want of some such familiar explanation as the following:—