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may be expected to continue; but if a rapid rise or fall takes place, the change may be expected the sooner, and will continue only a short time. Remark also, that while the particular height of the mercury must on no account be neglected, it is of still more importance to record how much, and in what time, it rises and falls.
In order, then, to interpret the predictions of the Barometer, it remains to enquire by what changes the atmosphere becomes heavy or light. The best understood are two:
1st, WIND.— Wind from the Poles is cold and heavy:
Barometer will rise.
Wind from the Equator is warm and light:
Barometer will fall.
2nd, Rain.—Moist air is light: Barometer will fall. Careful observation has shown, that, of the 3 inches (27} to near 31) through which the mercury fluctuates, a change from dry weather to heavy rain (the wind remaining the same) will occasion an extreme variation of about half an inch; a change in the direction of the wind from one point to the opposite, (say from N.E. to S.W. in this country,) about half an inch; and a change of the force of the wind from calm to hurricane, about two inches; so that the Barometer is much more a “wind” than a rain instrument; and this proves that the words rain, fair, &c. usually cut on Barometer scales, are not only useless, but mischievous, since they will frequently mislead the observer. Of much more value are the following brief but pithy rhymes, selected from that store of verse in which, like the ancient British Druids, our seamen clothe so much of their lore :
When the glass falls low,
Short notice--soon past. Bearing in mind the before-mentioned changes in the atmosphere which come within the scope of the Barometer, viz. Polar and Equatorial winds, and rain, the following summary should be remembered, and combined with the seaman's own experience.
IN THE NORTHERN HEMISPHERE.
The Barometer will RISE The Barometer will FALL for N.W., N., or N.E. winds, for S. E., S., or S. W. winds, (being Polar winds.)
(being Equatorial winds.)
For rain or snow from North quarter, the mercury may either rise or fall, the wind tending to raise, and the rain to depress it.
The Barometer stands highest for N.E., and lowest for 8.W. winds.
IN THE SOUTHERN HEMISPHERE.
The Barometer will RISE The Barometer will FALL for S. W., S., or S. E. winds, for N. E., N., or N. W. winds, (being Polar winds.)
(being Equatorial winds.)
For rain or snow from South quarter, the mercury may either rise or fall, the wind tending to raise, and the rain to depress it.
The Barometer stands highest for S.E., and lowest for N.W. winds.
The following Compasses will explain to the eye the connection between the Wind, the Barometer, and the Thermometer (or Heat-measurer.)
If the wind veer in the direction contrary to those laid down in the figures, it is a sign of more wind or bad weather.
The Barometer is invaluable in enabling seamen to know when they are approaching those devastating storms supposed to be “circular,” and known by the appellations Cyclones, Hurricanes, and Typhoons. It may be affirmed that the Barometer, in every case, gives warning of this approaching danger, provided its indications be read aright, although in the present very imperfect and unsatisfactory state of the law of storms,” there is much to be learnt about its motions previous to and during their operation. It is ascertained, that, within the Tropics, the Barometer has generally a very small daily range, and that its movements exhibit great regularity. There is a rise (amounting to about two-tenths of an inch,) terminating about 9 a. m., followed by a fall towards 4 p. m.; again a rise towards 10, p. m., and, finally, a fall ending at 4 a. m.; in fact, a flow and ebb of the mercury twice a day. The seaman should make himself familiar with this.
Observe, 1st,—That any interruption, even to a small extent, of this flow and ebb, portends a disturbance of the atmosphere.
Observe, 2nd,—That any sudden fall of the mercury, in tropical seas, is an unmistakeable warning of an approaching hurricane, and is a dangerous signal outside the Tropics. Sometimes an extraordinary fall of the Barometer precedes a hurricane, but not always. It is said that the nearer the centre of a “ circular” hurricane a ship gets, the lower the mercury but this does not seem to be always verified.
We warn seamen to watch the Barometer carefully in all known stormy or squally regions, but especially in the hurricane districts, near the West Indies, Mauritius, and the China Sea.
The average height of the Barometer is greater at the Tropics than elsewhere, being a trifle over 30 inches. On approaching the line from either side, it decreases till, near the line, there is an average depression of nearly inch, compared with the Tropics, (from observations by Sir J. Herschel.) The average for England is 29-94 inches. In the Southern Ocean a large mean depression seems to exist, for Sir J. Ross found the average at lat. 45° S. to be only 29•67 inches, and about lat. 51° S., 29.50 inches. It is of importance to obtain these quantities with some precision, for when the Barometer is at the average, fine and steady weather may more certainly be expected.
As a general rule, if a change of wind or fall of rain occur, sufficient to explain a previous change of the mercury, no further apprehension need be entertained; but always keep
falls ; the motions of the Barometer, for several days previous, in your mind; be very
cautious when the mercury is low, even though it be rising; and remember that it is better to be prepared for a storm many times too often than once too seldom.
MANAGEMENT OF THE BAROMETER.
Before fixing the Barometer in its place, it should be carefully examined to see if any air exists in the “ vacuum" above the mercury. Somewhat suddenly incline the Barometer, till the mercury runs to the top of the tube, with a slight jerk; if it does not, tap sharp, there is some air present. To eject it, invert the Barometer and gently strike it with the hand : this will drive the air-bubble up into the cistern.
The Barometer should be hung in gimbals, and so as not to swing too freely. It must be out of the sunshine, but in a good light for reading, as near midships as possible, and out of the reach of gusts of wind and sudden changes of heat.
In reading it at night, a piece of white paper should be placed behind the glass, (if the construction admits of it,) and a strong light behind the paper.
In making an observation, give a few gentle taps to the glass; set the index of the vernier to the upper surface of the mercury, being careful to hold the eye level with the top of the mercury, so as not to set the vernier too high or too low. If the top of the mercury appears arched, the vernier should be set to the top of the arch.
If the instrument is a “Kew Marine,” with the amount of corrections furnished, apply the correction to obtain the true height of the mercury. If, however, the glass is one of the ordinary kind, the owner should have it compared before leaving port, and after returning) with some standard, and the error of the scale ascertained. Ordinary ship Barometers, as frequently furnished, differ so much in the “set” of their scales, as to render observations by them valueless for comparison, unless this precaution be attended to.
The exact height of the cistern above the ship's water-line should also be known and recorded, and the Thermometer, attached to the Barometer, should be read at every observation.
Aneroid Barometers, when carefully constructed, are valuable substitutes for the Mercurial; but Wheel Barometers are valueless as scientific instruments,