development of magnetism in the stancheons, and other vertical iron of a ship. The chronometers were kept on board the whole winter, and their rates, preparatory to the polar navigation of the following summer, were assigned from the average of the four months immediately preceding her extrication from the ice, at an equal period of four months of navigation. The Hecla arrived at Leith, having experienced much bad weather in crossing the Atlantic, but, on comparing the four chronometers at the Observatory at Leith, their Greenwich time, employing the winter harbour rates, proved less than two seconds in error.

"On the arrival of the Hecla in the Thames, the chronometers were returned to Messrs. P. and F.'s house in London, when, after a month's interval, they were found to be still going at the same rate as in the Hecla whilst in the harbour of Melville Island."

Attention was first, we believe, drawn formally to the supposed alteration of the rates of chronometers on their removal on ship-board by the Rev. Mr. Fisher, who found that the rates of those in his charge were uniformly accelerated under such circumstances; and he assigned as the cause the magnetic effect of the iron, to which Captain Sabine, in the above, has so pointedly adverted.

Many persons well acquainted with the subject were of opinion at the time of the publication of Mr. Fisher's memoir, that, from the obvious inferiority of the chronometers which he used, no authoritative inference could be drawn from any anomalies which their rates might exhibit-an opinion in which we fully concur.

One of these chronometers had a rate on board of 3° 4"; but, on its removal to the Observatory, its rate was found to be 18s 2′′; and on being taken back again to the ship, the rate was found to be 6 5""; shewing a change in the ship-rate of 3s 1"".

Another of them, by a different maker, lost about 9s by its removal from the vessel to the shore; and a third, by another maker, still more. The variation in the shore rates is also remarkable, that of the first appearing to have been 85; of the second, 6'; of the third, 7s 2"; and of a fourth, 8°. And all these errors were noted in the short period of 17 days.

From the performance of such chronometers, it would surely be unsafe to draw positive inferences as to the effects of any generally operating causes on their rates of going.

Mr. W. C. Bond, of Boston, in America, a gentleman well qualified for the task, has taken much pains to ascertain whether there is any regular and systematic tendency in chronometers to change their rates when put on ship-board; and, in a paper published in the Transactions of the American Philosophical Society, he has given the result of experiments on a great number of chro

nometers.

When he had opportunity, he found the rate before the chronometer was sent to sea, and its rate after its return, and he took

the mean for the shore rate; and, dividing the change in the error which had occurred while the chronometer was at sea by the number of days, he obtained the sea rate.

The following extracts are the results from eighty-seven chronometers made by us, with their different numbers, and whose rates previous to the chronometers being placed on ship-board were accurately determined, and the same after their return from the voyage:

Number wherein the average shore rate differed from the ship-
board rate-and the difference

............

No. of No. of Days
Chron.

at Sea.

87

9

27 to 95

Number wherein there was no difference whatever

It is evident from these experiments, that there is no general tendency in these chronometers either to gain or lose at sea, on

Days at

Sea.

their land rates; as it appears from the above, that, out of eightyseven trials, thirty-nine gained on their rates, and thirty-nine lost on their rates: the remaining nine made no variation whatever.

Mr. Barlow, of the Royal Military Academy at Woolwich, whose attainments in science, skill as an experimenter, and discoveries on the laws of magnetism in particular, are known to all scientific men, took up the subject on the appearance of the Rev. Mr. Fisher's paper, and published the result of his inquiries in the Transactions of the Royal Society. He found, indeed, that chronometers were influenced by their near proximity to masses of iron; but instead of the rates of those which he tried being accelerated, five of the six which he used were retarded, and the acceleration of the sixth was doubtful.

We may be excused for stating, that the one least affected was made by us, and it was constructed on the same principle as the four mentioned above by Captain Sabine, and whose performance, under such extraordinary circumstances, was to us a subject of gratifying remark.

That a material effect on the going of a chronometer would be produced by applying a powerful magnet to it, we have no doubt, as the magnet would then operate as a disturbing force with all the advantage of proximity, but to infer from thence that the rate of a chronometer must necessarily be affected by its removal within the sphere of the ordinary magnetic influence existing in a ship, appears to us not more legitimate than to infer, that, because a chronometer will stop if put in the fire, it will necessarily go ill in the ordinary temperature of a sitting-room.

We are far from imagining, that, because so much has been done for the improvement of chronometers, there is nothing left to be desired; and we shall rejoice unfeignedly at any suggestion which may enable those who are engaged with the delicate task of constructing them, to arrive at this, and by more simple and certain means. Our object in writing to you on this occasion, is to convince those whom it chiefly concerns, that the errors, and causes of errors, on which your respectable correspondents have animadverted, cannot, in the present state of chronometrical science, have any appreciable effect in practice.

We are, Sir, your obedient servants,
PARKINSON & FRODSHAM.

'Change Alley, November 14, 1833.

IV. MITCHELL'S NEW MOORING.

London, Nov. 21, 1833.

SIR-I have been much gratified by learning that my new mooring, as connected with my lately patented dock, has been considered

by you as deserving public attention, and I have much pleasure in giving you a more detailed account of it.

The annexed drawing will explain its construction. It is on the principle of the screw, the spiral thread being extended to a broad flange, with little more than one revolution round the central shaft. This flange, when forced round by means of a long shaft, adapted by joints to the depth of water, insinuates itself into the earth, until a firm hold is obtained, when the long shaft is withdrawn, leaving the mooring at the required depth, with a strong bridle-chain attached.

Fig. 1. The screw mooring, as prepared for use.

a. A spiral, or screw flange of about one and a half turns, having a hollow cylindrical centre cast of iron in one piece.

b. A wrought iron spindle, passing through the cylindrical socket of the screw flange, somewhat taper, and when driven up, fixed thereto by a key which passes through both. A square head to receive the key for turning it into the ground.

d. A collar of wrought iron, fitting so as to turn freely on the upper part of the shaft of the spindle below the shoulder.

e. A shackle turning upon two journals of the collar.

Fig. 2. Shews the upper surface of the spiral flange. The spiral makes about one turn and a half, and curves in towards the centre, at both terminations, as shewn, having its edge somewhat sharp, to facilitate its entrance into the ground. The hole through it is a little conical, corresponding with the taper of the spindle, so that the latter may be driven firmly in before the key is introduced.

Fig. 3. The spindle.

Fig. 4. The collar and shackle. The shackle is fixed to the spindle by means of the loose collar, in order to prevent the dragging round, and consequent fouling of the chain, whilst the spindle is being turned in or out of the ground.

These moorings may be used wherever the water is not of extraordinary depth, unless the ground be rocky. In soft ground they may be sunk considerably below the surface. I have myself placed them twenty feet beneath the bed of a river. However, where the bottom is firm, or what seamen call good holding-ground, a depth of six or seven feet is generally sufficient.

The size of the mooring must be regulated by the nature of the ground, and the resistance required.

I have used them from two to five feet in diameter, and there is nothing to prevent their being cast of a much larger size, should it be found necessary.

Allow me now to compare my mooring with the two at present in general use, and I will point out some circumstances which I conceive will prove the advantage of mine.

One of the two methods I allude to consists of a broad flat stone being sunk two or three feet into the ground with a bridle-chain. To this mode it is objected, that in shallow water, vessels frequently receive injury from it; and, besides, the excavation for receiving the stone is almost impracticable in more than three, or three and a half fathoms water. The other description of moorings is formed of a strong chain stretched along the ground, and secured at its extremities by mooring-blocks or anchors, the vessel swinging at a bridle-chain near the middle. But the vast expense of this apparatus, the foul bottom which it makes-ships' anchors often catching in the ground chain, and thereby occasioning much labour and difficulty-and the fact, that in shallow water the mooring-blocks are as dangerous as rocks; all these are objections which it would be well to remove. Now, my mooring-machine, I may venture to say, is liable to none of the above objections. It is necessarily so far below the surface of the ground, that the ship's bottom cannot possibly come in contact with it, as in the case with the stone mooring; and the necessity of ground-chains and mooring-blocks being