페이지 이미지

raervoir of flame, which was propelled from its volumnious bed by some invisible but omnipotent agency, and threatening to fling its fiery ruin npon every thing around. In some parts, however, of the pitchy vapour by which the skies were by this time completely overspread, the lightning was seen only occasionally to glimmer in faint streaks of light, as if straggling but unable to escape from its prison—igniting, but too weak to burst the impervious bosom of those capacious magazines in which it was at once engendered and pent up. So heavy and continuous was the rain, that scarcely any thing save those vivid bursts of light, which nothing could arrest or resist, was perceptible through it. The thunder was so painfully loud that it frequently caused the ear to throb; it seemed as if mines were momentarily springing to the heavens. The surf was raised by the wind and scattered in thin billows of foam over the esplanade, which was completely powdered with the white feathery spray. It extended several hundred yards from the beach: fish upwards of three inches long were found upon the flat roofs of the houses in the town during the prevalence of the monsoon, either blown from the sea by the violence of the gale, or taken up in the wateripouts, which are very prevalent in this tempestuous season.

The annexed sketch, from Colonel Reid's valuable work on the Law of Storms,* represents the condition of the mouth of the Hoogly in a hurricane, from a painting by Iluggina.


The Influence Of String Tides On The Weather.

Interesting facts with respect to the fluctuations of the weather are constantly occurring, but unheeded by the majority of persons who have

• We are glad to perceive that this work is now going into a second edition.

leisure to make useful observations: yet, were these fluctuations studied with the attention they deserve, it is highly probable that they would afford us the means of often anticipating the changes, and of regulating our movements accordingly. To be enabled to do this, it will be admitted by all, would be a very comfortable reflection.

One of the most prominent of these variations is that of the spring flood-tides bringing from the ocean a vast collection of aqueous vapour; a fact known to fishers and pilots long before it was verified by Colonel Capper, on the Welsh coast, there can be no doubt. The Colonel also considers that an increase of the electric-fluid takes place; and he suggests that, the changes of weather hitherto ascribed to the direct influence of the moon may be imputed to the circulation of that fluid, and which is, in a great degree, immediately occasioned by the flux and reflux of the tides. That the vapours, thus brought in by the spring tides, are also attracted by elevated land, and by electrical action of the clouds, become precipitated, seems to admit of little doubt. The circumstance of the Scilly Isles, although surrounded by a humid atmosphere, not being so subject to rain as the main shore, which is more elevated ; as also the east coast of England, which is comparatively low land, and well-known to be considerably dryer than the western and south-western coasts, may be advanced as corroborative of such operation. In all seasons during the continuance of the spring tides, in the vicinity of the estuary of the Severn, and probably the same may be the case on all the hilly parts of the western and south-western coasts of England, when westerly and south-westerly winds prevail, pluvial discharges are frequent; and we have been often surprised at finding these so local, in many instances occupying a very limited space on either side of the river Avon. On the turn of the tide the weather generally clears up.

An incidental remark of Mr. Rootsey, at the late meeting of the Philosophical Society, held at Bristol, is worthy of consideration. That intelligent gentleman observed, that, "in variable weather, the crisis of the day was always to be looked for at the changes of the tides. The tide-wave, when of the enormous magnitude with which it reached Bristol, (fifty feet,) must alternately lift up and let down the atmospheric column which stood upon it, and thus gave rise to changes in the barometric state of the column, which everybody knew caused the other changes, or, at least, preceded them."

For several years past it has been noticed that both rain and an increase of wind (westerly) generally took place during the flood of spring tides, and that the first often ceased, and the latter usually lessened as the tide ebbed; but the reason assigned (viz., attraction of the fluent water) for these coincidences was different from that above given, which seems to be one of the causes of an increase of wind.

May not the gravitating pressure acting downwards, combined with the upward forcing of the incumbent atmosphere, have the effect of accellerating the progressive movement of the westerly wind, or of creating a motion of the air when it may be calm, under certain conditions!

By the vertical pressure of the tide, would not the condensation of the column of air be increased? And this, if so happening when already in motion, would there not be a tendency to increased action ?* As the strongest principle of water is, to maintain its level, so in the fluid air the principle is as strong to be at rest; and this state in either, we all know from observation, can often be only fully accomplished by a violent struggle.

But, as an increase of wind, although generally occurring, is not an invariable consequence of the tidal operation, it must be presumed that there are auxiliary causes in action at the time, or preceding it, to produce such an effect.

The fact, however, that an increase of strength of the westerly winds, when attended with moisture, is generally coincident with the rise of spring tides, and as generally subsides again with the ebb, is too remarkable to be questioned. We need not, therefore, be at a loss to account for those temporary gales so commonly experienced in the locality of the Bristol Channel at such periods. It may be well for those mariners who navigate that arm of the sea to study the subject with care; for, assuredly, to be enabled to predict the coming and probable duration of a storm is an advantage not to be despised near coasts so full of dangers.

Indeed, this view of the subject might be extended to the wide ocean, and the same effect, proportioned to the difference of rise, may happen as the great tidal wave advances along the Atlantic, modified, of course, by the state of the weather, and principally regulated by the hygrometric condition of the air.

If it should be calm, a movement of the air in the direction of the tidal wave as it rises under any parallel, would probably be the result. If a light wind should be blowing, it would, perhaps, in like manner, be increased to a fresh breeze, and so on to a gale. In this way we may readily account for those sudden but temporary increases of wind so often experienced at sea. The great extent of the wave may, in some measure, make up for the want of elevation, and the effect, although it may not be so considerable when compared with that which takes place when the waters become fluent, and the vertical rise augmented, may not be the less certain. At all events, the suggestion is offered as one of the probable causes of a sudden increase and short duration of wind in the ocean, as whatever disturbs the equilibrium of the atmosphere must create a movement of the air.

The process appears extremely reasonable, under certain conditions, and those who will call to mind the compressive action of a bellows may perhaps be disposed to reconcile the observed effect with the given cause.

In pursuing the investigation of the subject, the following inferences were drawn from observations already made. When the wind is at any point between south-west and west of whatever strength it may be previously to, it will generally be found to increase with the spring flood tides, if these should be attended with moisture, and to subside again with the ebb; the increase of the wind commencing some hours before

• Whilst this process, if it be admitted, is going on over the water, the heat and moisture brought in from the ocean would increase the expansibility of the air as it reached the land, and account for the violent gusts which are experienced at such times.


high water, and the decrease a variable time after the turn of the tide.*

That, when the wind is between those points, and it happens to be dry clear weather, little increase of the strength of the wind may be expected to follow, which would seem to point to the hygrometric state of the air as the regulator.

If the wind be at south, and attended with wet, it will increase with the flowing tide, but the ebb seems to have little effect in diminishing its strength, and the crisis may be looked for as the stm attains his meridian altitude; or, if a shift of wind takes place to the northward, after that has passed without alteration.

If the wind be blowing across the channel (Bristol) during spring tides, it becomes very unsteady, vacillating several points to and fro, or veering and backing, according to sea-languge, incessantly, with alternate rapid increase and decrease of its strength. If it should be oblique in its direction before the tide commences in the ocean, it will probably veer to the westward until the ebb takes place.

These are the facts as far as they yet have been observed; and without meaning to determine the philosophical deduction for these phenomena, the following reasons are offered why the alternation spoken of should reciprocate with the tides :—

1st. That, the points named are in accordance with the lines pursued by the tidal wave up the channel—that is east and north-east.

2nd. That, a disarrangement of the air incumbent over the channel water takes place by the vertical rise of the tide. . 3rd. That, from electrical changes which supervene on the insprirjgirig of the sea and land clouds, rain is precipitated.

4th. That, an accession of heat is brought in from the ocean " upon the wings of the wind," as also aqueous vapours in excess.

5th. That, moist air is increased in expansibility by heat.

It must be borne in mind that these remarks are made at a position being about three miles direct from King-road, which is nearly at the head of the Bristol Channel.

Multiplied observations are necessary to establish the given facts, and the inferences drawn from them. Naval officers who may reside at the following places,+ are therefore invited to contribute to the desired end, assuring any who may be labouring under that most vexatious of all vexatious distempers, termed in common parlance, the" Blue Devils," that they cannot find in the whole Materia Medica, a recipe more potent for dispelling the affection, or affliction, than that of renewing their acquaintance with Doctor Fresh Air!

A West Country Coaster.

On The Action Of The Wind.

Sir John Herschel remarks that, " meteorology, one of the most complicated but important branches of science, is, at the same time, one to

• The tides here are very irregular, differing sometimes as much as four hours from the table.

t Barnstaple, Bideford, Penzance, Falmouth, Plymouth, Chepstow, Newport, Swansea, Milford, and Fishguard.

which any person who will attend to plain rules, and bestow the necessary degree of attention, may do effectual service."

There is much encouragement given in these few lines to the am all fry who possess a penchant for science, without the acquirements of the philosopher. If, therefore, naval officers who may have leisure to attend to such matters, would but enter upon the subject with zeal, not only in this country, but wherever they may happen to reside or sojourn in distant parts of the world, a fund of useful information would be the result; whilst in the very undertaking they would be advancing their own happiness, by being rationally employed—be still aiding theif noble profession—and conducing (no matter in how trifling a degree) to the advancement of science. Having myself paid some attention to, and reflected much on the subject of our present paper, I have ventured to embody here my remarks, and now submit them, Mr. Editor, to yourself, and to the readers of the Nautical, with the hope that, if any have made similar observations, or entertain the same opinions, they will favour us with them through the medium of your pages.

1. The long received opinion of winds blowing over a large tract of continent, or ocean, on a rectilinear course, it is highly probable, in the course of future observations, will be greatly modified.

That the operation takes place cannot be doubted, but not so generally as is supposed: indeed, to speak strictly, such opinion would amount to a physical impossibility, as to any extent, the earth being spherical, the currents of air must necessarily form curved lines: but, taking the area of the horizon, or the ocean between any two points as a plane, the following remarks will allude to an horizontal curve. •

That westerly winds, (leaving aside the perennial winds of the tropics,) do extend from continent to continent unbroken, and often undeviating across the Atlantic, is true beyond dispute, the fact being proved in thousands of instances, as also to a great extent in the Southern and Great Oceans, if, indeed, they do not entirely encircle the globe in the other hemisphere.

But there is reason for considering that winds frequently pursue a curvilinear course. This may probably be the case when two currents of air happen to meet obliquely, the strongest turning aside the weakest, so that a ship crossing the space occupied by the stream of air, would experience shifts of wind as she moved along the curve. Such an operation would serve to account for the remarks (and the frequency of such remarks is one of the reasons which give rise to the opinion) so common in the journals of seamen, "of the wind shifting round the compass," but which often amount to a vague and incorrect mode of expression contrary to fact, the extent of the variations not embracing the quadrant of a circle. It is not denied that the wind shifts round so as to complete an apparent circular movement, but oflener six or eight points only may be taken as the amount of the variations.

When the strength of two winds thus opposed may be equal, or nearly so, then they may combine and pursue one direct course: for instance,—if a north-west and a north-east wind meet and flow together, a south direction would be given to the combined currents. A ship, consequently, going with either the north-west or north-east vein, on arriving at the angle of incidence, would experience a sudden shift of

« 이전계속 »