Графични страници
PDF файл
ePub
[graphic]
[ocr errors]
[ocr errors]

29. 00 28. ool 30.00

in 30. 90 Reading

10 20 30 40 50 70 30 90 20 30 40 50 60 70 SO 90 20 To 30 40 160 70 SO

NOVEMBER Inches. 5 9 10 11 12 13 14 15 16 17 18 192021222324252627282930 Inches.

128.00 29.00 130.00

in Reading

20 30.90

30 150 40 60 180 70 10 10 20 30 40 50 70 60 80 90 20 30 40 50 60 70

[ocr errors]
[ocr errors]

very high, and was above the average. On the 7th day it decreased a little below the average ; on the 8th the lowest reading. in the month took place, viz., 29.76 inches; and from the 9th day it was constantly above the average to the end of the month, being mostly above 30 inches, as shown in the diagram. The weather was unusually fine throughout the month. The bighest reading in the month took place on the 23rd. There was a little decrease in the readings at the beginning of October, but on the 3rd day the reading again rose above 30 inches, and began to decline on the 5th, continuing to decrease till the 11th. Up to the 10th day the air had been mostly calm, and fog had been prevalent, but on this day the wind began to blow strongly from the W. and S.W. On the 11th the readings began to increase, and

, frorn the 12th the winds were light. A maximum barometer reading was reached on the evening of the 15th day. On the 16th the barometer readings decreased rapidly, and the wind increased in strength. A minimum reading of 28-89 inches was reached by 9 p.m. on the 18th, with strong wind blowing. Unsettled weather followed this, and the barometer readings oscillated up and down, as will be seen by reference to the diagram, and the wind was frequently very strong, reaching 20 lbs. on the square foot on the 25th day. From this time to the end of the month, the oscillations of the barometer readings were very remarkable ; a decrease of 3 of an inch was followed by an increase to the like amount; and then in very unusually quick succession, another decrease and increase to almost the same amounts, these very large and rapid changes extending over very considerable tracts of country and sea. The large numbers at the bottom of the table on those days will show how strongly the wind was blowing.

From this time there is a general bold and continuous increase in the barometer curve to 304 inches on the 12th of November, with mostly N. and N.E. winds blowing, and sometimes strongly. From the 13th day set in decreasing readinys, which with checks, as shown in the diagram, fell to 28.82 inches on the 22nd day. At Liverpool Observatory, on this day, the reading was as low as 28-31 inches, as recorded by John Hartnup, Esq., the Director of the Observatory; and from this time to the 28th day, there was succession of disastrous storms, some of the effects of which were detailed in the newspapers at the time.

If we collate the numbers in the lower part of the chart together with different readings of the barometer, we shall find that when the readings exceed 30 inches, the average daily Horizontal Movement

of the air was about 130 miles per day; about 30

ditto

160 between 29.5 & 30.0 in. ditto

210 29.0 & 29.5 in. ditto

260 below 29 inches ditto

320

[ocr errors]

thus showing that whenever the barometer reading has been above the average, the wind has been moderate, and that as the barometer reading decreases, the motion of the wind increases, and is the greatest at the lowest readings.

It is impossible to urge too strongly the necessity of care when continuous declining readings are proceeding. The barometer may be almost neglected by the sailor when its readings range above the average; but when they descend below the average, it is a warning which ought never to pass unheeded ; and when the depression is sudden, it is the sure and certain warning of the approach of storms. At the beginning of last month, when the unhappy ship London put to sea, the barometer was very unsettled indeed. Such signs no sailor ought therefore to neglect; yet they are neglected; and what is more sad still, too many barometers, as we before said, sold as marine barometers are totally unfit for such an important purpose. During the last few years I have seen many of these instruments, and I have found them imperfect in numerous ways.

Some of these defects I may enumerate here :

Firstly. The upper portion of the tube is too small.

Secondly. The relation of the interior bore of the tube to that of the cistern is neglected, so that the rise of the mercury in the cistern, caused by a fall in the tube, is not allowed for on the scale.

Thirdly. The lower portion of the tube is generally too much contracted in the bore.

Fourthly. The cistern is too small to contain the mercury when the barometer reading is low.

Fifthly. The careless cleaning out of the internal bore of the tubes, and imperfectly freeing them from damp.

The most serious of these errors is the third. It is well known that the tubes of marine barometers ought to have their bores contracted so as to steady the motion of the mercury when the ship is in a heavy sea. This, in some instances, is so carelessly done, that it has not the desired effect; but in most cases it is carried to a great excess. For instance, in some instruments the mercury will take from forty minutes to an hour to pass through the first inch after suspension, and many are so sluggish in their action as not to indicate any change in less than twenty minutes.

This unnecessary contraction is very frequently the cause of entirely stopping the action of the barometer, as the minutest particle of foreign matter, or even the oxidation of the mercury itself, which, as an impalpable powder forming on the surface of the mercury in the cistern on moving the barometer, is displaced, and this in its ascent to float on the top of the mercury, chokes and closes the fine bore of the tube hermetically, rendering the instrument perfectly useless; and this is often not found out till the ship has left for sea. A barometer, therefore, acting well till disturbed, may be quite useless afterwards.

The fourth-named error is also one of great gravity; and for this error low-priced inanufacturers are far more to be censured for their culpability than for their ignorance or carelessness—the size of the cistern is reduced to enable them to save a quarter of a pound of mercury. This great desire to cheapen instruments is most serious, as the indications of such instruments are absolutely deceiving, and at a time when the changes are of vital importance to the sailor. In many such barometers the mercury will not fall below, even if so low, as 29 inches, although a standard at such times may read 28.5 inches, or even less. Nor will such an instrument show any change till the mercury rises above 29 inches ; that is, the barometer says the mercury is stationary and steady at all times of the greatest phases of storms, and when every change should be instantly known by the sailor

We fear that Master Mariners care nothing, as a rule, for barometers and scientific meteorology. They don't believe in them; and the reason is plain to anybody who walks the East-end. Those worthless things hung up for sale in the shops of pawnbrokers, ship chandlers, and slop sellers, are the accursed shams that cost life at sea. Captains buy them and fine them worthless; then they give the thing up altogether, except in the way of a general “glance at the glass.” They take good chronometers and good sextants to sea because these instruments can be tested, but with barometers they too often decline to trouble themselves. Of course there were such things on board the London, but they are not half sufficiently respected and understood. There should be a distinct examination of sailors in meteorology. Were the barometer thus tested, and thus explained and understood, skippers would as soon start without rudder or sails as without that moni. tor; and they would obey it, leaning to the side of too much caution rather than too little. Underwriters should not insure vessels unless provided with first-rate weather instruments; nor should it be the rule, as it is, that captains should be allowed to rely upon such guides or not as they please. It is not because occasional perturbations, more complex than usual, baffle the reckonings of science, that meteorology should be ignored--four times out of five the storm can be safely foretold. We know that good barometers are taken to sea, and that such companies as the Peninsular and Oriental have saved magnificent ships by them. We also know that an instrument may be tested at Greenwich or Kew. But we want mariners to attach due importance to these all-important aids. With the extending of educated experience, our weather wisdom should be surer than it is; now, to-day, no fishing-village that sends a couple of trawlers out should want a first rate barometer, and no captain, if he can possibly help it, should put to sea in the teeth of storm signals.

« ПредишнаНапред »