401

VOYAGES AND MARITIME PAPERS.

I. THE TIDE GAUGE AT SHEERNESS.

THE drawing at the commencement of this number presents a plan of the Tide Gauge in H. M. Dock-yard at Sheerness, which, being the most perfect machine of the kind that we know of, a description of it may not prove unacceptable to our

readers :

A-Is the elevation in profile of the tide-gauge house, which stands on a small wharf at the S.W. angle of the Boat Basin, open to the sea.

B--A front view of the same, shewing as much of the machinery as can be seen from the door-way.

C-A wooden trunk, two feet square, and twenty-six feet long, reaching to the bottom of the basin; the lower end bored with holes, to admit freely the ebb and flow of the tide, without being affected by the waves.

D-A long wooden tube, six inches square, and twenty-six feet high, from the top of the wharf, to protect the gauge-rod. E-A copper float, or buoy, fixed at the lower end of F-A moveable gauge-rod, of light wood, one foot and a quarter square, and twenty-six feet long, guided by friction rollers at the top, and sliding inside the trunk against a fixed scale, divided to tenths of an inch; the divisions on the scale being numbered upwards, on the right-hand side of it, to shew the rise of the tide, and on the left-hand side, downwards, to shew its fall. Between the scale and the moveable rod, on each side is a groove, containing a sliding vernier bearing a catch, G and H, both of which project over the face of the rod: the catch on the right hand,

G-Is caught by a projecting nail, I, on the lower part of the rod, and is carried up during the rise of the tide while that on the left, H-Is brought down (by another nail, K, fixed on the upper part of the rod) as the water falls; the distance between the two nails, or I and K, being exactly fourteen feet.

When the sea is at its mean level at Sheerness, which has been found from the mean of several years' observations, (and which corresponds exactly with the mark "eighteen feet" at the entrance of the basin,) the O mark, or zero, on the rod is made to agree with seven feet on the two scales, as is represented in the plan. Now, should the tide flow to cause the nail at I to raise the catch G with its vernier, to 8 ft. on the scale; and again fall, till the nail K on the upper part of the rod bring the catch H, with its vernier, on NO 8.-VOL. I.

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the left-hand side down to 8 ft. it is evident that the tide ha om ranged sixteen feet; which, with the time, the observer must note dal down, and set the verniers afresh.

Fill

Again, supposing the rising tide to carry the catch G up to ( 11 ft. on the right-hand scale, and the falling tide to bring H down to 5 ft. on the left-hand scale, the tide has still ranged sixteenly feet; but it is plain that the rise has far exceeded the fall, and that either a strong wind or some other cause has prevented the ebb of the water. This arrangement of the indices is perfectly s simple, and no mistake in reading off can be made by the commonest observer.

Yet, notwithstanding the correctness of this gauge in giving the rise and fall of tide, it requires an observer at high and low water to watch for the time. By night, no attendant is on the spot; by day, too, he is sometimes absent; and even when present, the most watchful observer often cannot tell the precise time from five to twenty minutes; as the water is at times stationary, or nearly so, for more than half an hour; moreover, sometimes it falls a few inches, and rises again. To meet these difficulties, it occurred to the civil engineer at that dock-yard, Mr. Mitchell, to cause the tide-gauge to register itself, which is done in the most ingenious, and at the same time in the simplest manner, by the application of a little machinery, as follows:

A sheet of paper is divided by lines into twenty-four equal parts, for hours, on the scale of 0.5 of an inch to an hour, and these lines are crossed at right angles by twenty-four other lines, to shew the range of the tide in feet, on the scale of 0.4 of an inch to a foot. The paper (S) so prepared is wrapped round a cylinder or roller of wood, the circumference of which is exactly equal to the twenty-four divisions of time; the roller is then supported horizontally, and so connected with a clock, (R,) that it makes one revolution in twenty-four hours.

By means of three wheels, (LNO,) the vertical motion of the gauge-rod is communicated horizontally to a sliding bar of brass, Q, (in which is fixed a pencil, P,) causing it to traverse across the paper as the tide rises and falls parallel to the axis of the cylinder; the pencil at the same time tracing a line indicated by the

The principle of this tide-guage is the same as that mentioned in Mr. Lubbock's valuable paper on "tides" in the Companion to the British Almanack for 1830; but the plan and execution are solely those of Mr. Mitchell, of the Dock-yard.

The vertical motion of the guage-rod is thus communicated: L-Is a wheel twelve inches in diameter, with a grooved edge to receive M, a small cord which passes round L, in the direction of the arrows, and is attached to a spring at each end of the gauge-rod; which springs, as well as the tightening pulley, T, always keep the cord at an equal tension; as the rod rises, the cord will make the wheel revolve in the direction of the arrows; as, also, another wheel, N, of four inches diameter, on the same axis which communicates motion by an endless cord to O, a wheel of twelve inches diameter, on the axis of which is a brass pinion which causes the rack, or toothed bar, Q, with the pencil in it, to move to and fro, as the tide rises or falls, in proportion as the divisions on the paper are to each foot of the gauge-rod.

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Combined motions of the clock and the gauge-rod: this line is the zidal curve, and, as far as the nature of the materials employed will admit of, is strictly correct;* entirely obviates the necessity of an observer, excepting once a fortnight, at full and change of moon, to replace the sheet of paper, and shews, on inspection, not only the rise and fall, and time of high and low water, but also every, even the most trifling irregularity which may have occurred at any time during the lunation.

Such is the plan of the self-registering tide-gauge at Sheerness; a wind-gauge will shortly be added to it, that will shew both the force and direction of the wind, and thus render it the most complete thing of the kind in Europe. May we not hope, now that such attention has been shewn to the subject of " Tides ;" in proof of which we may allude to the able "Paper on Tides" read by Mr. Lubbock at the late "Solemn Session of Science" at Oxford; may we not hope, that ere long we may see such establishments formed at all our principal sea-ports; the diagrams and observations from which would enable us, by comparing them, to throw some light on the irregularities of the tides, irregularities which are proved by the following instance not to depend solely upon the direction of the wind: e.g. the greatest spring-tide yet measured at Sheerness, viz., 20 feet, and the least range of neap, 7 feet 94 inches, both occurred with a strong S.W. wind.

The expense of an establishment similar to that at Sheerness would not be great; from 50 to £100 would complete the tidegauge at Portsmouth, and erect one at Plymouth and surely such a sum is not worth consideration, more especially in a country rendered by its sea-girt shores so essentially maritime as Great Britain. Indebted to the ocean, as we are, for our very existence as a nation, surely it ill becomes us to be indifferent, or inattentive, to the striking phenomena which that ocean daily presents to us, in the unceasing ebb and flow of its tides.

There are other means by which persons who, having the opportunity, might with great facility be enabled to register the phenomena of the tides, and the following is perhaps as simple and certain as any.

Let a leaden pipe be obtained, at one end of which a rose similar to that of a watering pot is to be fixed. Let this, as the outer end, be placed in the water, sufficiently deep to be always well below the level of the lowest spring tides. In placing it, care should be taken that no weeds, sand, or mud should prevent the water from passing through the rose into the pipe; and a situation where the water is deep should be preferred, so that the rose, with its face downwards, while it is below the level of the low-water of springtides, should be at the same time well above the bottom. The pipe

* A check is always kept by an observer, and in no case has this machine ever been at fault since it first came into use in September, 1831.