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HINTS TO CAPTAINS AND MATES ON
THE USE OF THE BAROMETER.
While in some localities the pressure of the atmosphere" has been known to equal the weight of a column of pure mercury nearly 31 inches in height, in others (at the sea-level) it has balanced only 27] inches. Thus we see that the atmosphere possesses in itself such varieties of pressure, as to render it impossible to remain at rest. The exchange of masses of air, so necessitated, is constantly going on, either gently by means of moderate and fresh breezes, or violently through the agency of storms. Originally generated by the Sun's gradual action, (probably aided by less understood causes, lunar and magnetic,) atmospheric changes are accomplished gradually, and hence may be “predicted,” if proper precautions are taken. To predict weather-changes, two means must be employed. 1st,- Instruments, such as the Barometer, Thermo
meter, &c. 2nd, -Appearance of the sky, clouds, &c. The two should be compared with each other, and neither should on any account be neglected.
THE BAROMETER AND ITS USES. A Barometer is simply a machine for weighing the pressure of the atmosphere. This it does by balancing a column of pure mercury in an upright glass tube. The tube is quite open at the lower end, where it stands in a cistern partly filled with mercury, to which the atmospheric pressure has free access. The upper part of the tube, above the mercury, is emptied of air and effectually closed. Thus all the pressure is at the lower or cistern end. Hence, wben the atmosphere is heavy, the mercury will be pressed up the tube, or “stand high ;' and, contrarily, when the atmosphere becomes less weighty, the mercury will “ fall.” If, therefore, the Barometer is rising, we infer that the atmospheric pressure is increasing, and if falling, that the pressure is lessening. Further, if the glass rises or falls slowly, the change thus occurring with deliberation 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.
Barometer will fall. 2nd, Rain.—Moist air is light : Barometer will fall. Careful observation has shewn, 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 mis. chievous, 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.,. N. E. winds, for S. En, S., 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 S.W. winds.
IN THE SOUTHERN HEMISPHERE,
The Barometer will RISE for S. W., S., S. E. winds, (being Polar winds.)
The Barometer will FALL for N. E., N., N. W. 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 exbibit 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 101 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 falls; 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 1 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 the motions