joined to the bottom of the former by riveting and caulking, thus forming a cubical vessel, 6 feet in height, by 4 feet in breadth each way, and open at the bottom. Round the upper square of the head, bars of iron, 2 inches broad and a quarter of an inch thick, were riveted, and others were placed down each side of the corners, from the head to the lower edge, which was also strengthened in the same way as the head. In the inside, at the upper corners, were diagonal bars to afford additional support against the pressure of water when the bell was suspended. Slings, made of the Lightning's top-chain, with shackles, were attached at each corner of the head, and the other extremities were united at the point of suspension by a chain cable shackle. For the purpose of weighting the bell, three loops of bar-iron were placed on each side of the lower half, through which a sufficient quantity of chain-cable was rove, with the addition of four large pigs of ballast, one fixed in each corner, in the inside, to sink it. At 18 inches from the lower edge in the inside, were two bars of iron, to answer the double purpose of strengthening the bell, and supporting two seats for the men to sit on; and across the centre of the bottom at the extreme lower edge, was a flat bar of iron to rest their feet on, which was removable at pleasure, to be put out of the way when the bell was at the bottom, so as not to obstruct their work. On the upper part, in the inside, were numerous hooks, for the purpose of suspending the various implements for boring rocks, digging, &c. It was lighted by six patent illuminators, two on the top, and one on each side. When weighted, it weighed about four tons, but it was afterwards made considerably lighter.

This bell was worked from a davit or crane fixed in the stern of a launch, which was a service of great labour and danger, the violent surging of the boat with the top weight of the bell on the davit frequently endangering its being swamped; and, in order to remedy this danger, Captain Dickinson greatly improved the bells subsequently constructed, by loading them with pigs of ballast only, placed within an iron frame, and merely wedged in, so that, in the event of the wind suddenly changing, they could be easily removed, and the bell be rendered so light that it could be shifted in a few minutes, by a small tackle, from the davit to the centre of the launch, the top weight being thus relieved, and the ballast available for the boat.

While the bell was worked in this manner, the advantage of the wrought-iron material was made manifest, the bell frequently oscillating to the extent of 10 or 12 feet, and it is more than probable that, under the same

51

circumstances, a cast-iron bell would have been split, and the lives of the men lost.

In consequence of the labour and danger of towing the launch, at the close of each day's work, nearly a mile along the coast, and through a narrow strait, subject to violent currents into the still water of a bay, on the beach of which the crew of the Lightning were encamped, Captain Dickinson devised the construction of a derrick, of 158 feet in length, made up of twenty-two separate pieces of spars recovered from the wreck of the Thetis. The derrick was stepped in an excavation in the rock within the cove, a few feet above the water's edge, and supported at its head by a cable made fast to the rocks above, at the height of 150 feet, with various other stays, whereby the outer end of the derrick was raised to the height of about 40 feet above the sea. The summit of the cliff was levelled, and holes were worked in the granite wherein capstans and crabs were fixed; the crabs having been formed out of the stumps of the topmasts saved from the wreck. A stage was suspended from the derrick, from which a divingbell larger than the others, but of a similar construction, was successfully worked. By all the contrivances, in which Captain Dickinson displayed consummate professional skill and ingenuity, turning all his disposable materials to account, and meeting each difficulty as it arose, no less than 18ths of the treasure, and a large quantity of government stores, were recovered.

The whole account of the operations forms a most interesting narrative of patient endurance, of fatigue, sickness and hardship under difficulties so extraordinary, that they would have been deemed insuperable by any but British sailors under the direction of a most intelligent and enterprising commander.

[The Society awarded to Captain Dickinson their Gold Isis Medal.]

THE CASE IN LEVERAGE.

Sir," H. H." now admits, (p. 454, vol. xxxix.) that the explanation I offered of the phenomenon (not anomaly) of equilibrium taking place in the case of a balance where the scales are loaded with unequal weights, is "perfectly correct," although he formerly designated that explanation as

an

"imagined" one. So far, therefore, I am satisfied. But there are one or two points in his last communication, to which attention may be advantageously directed.

1. "H. H." complains that in my communication (p. 325) I "occupied unnecessarily much space in showing the probability

of the coincidence of the centres of gravity, and suspension of a pulley." It is long since the observation was made, that it is much easier to give currency to error than to counteract it. "H. H." asserted that the centres in question could not possibly coincide. The shortest way of meeting such a ridiculous assertion would doubtless have been to contradict it, and to re-assert, as I had before done by implication, that two centres could coincide. But this would have been a less satisfactory way of meeting "H. H.'s" assertion than the way I adopted, which was to prove that two centres could coincide. And I also took occasion to show that the probability of what, for all practical purposes, may be regarded as coincidence, was very great indeed. In doing this I occupied, I think, no unreasonable space. But there is no pleasing some people; "H. H.," I dare say, would have preferred that I had left that point alone.

2. But "H. H." now says that his assertion, that the coincidence in question "could not actually exist," was only a supposition, made for the purpose of rendering his explanation general. Never before, I will take upon me to say, was a supposition made in such terms. Why he tells us in so many words that we are tied up to it, inasmuch as a different state of things cannot actually exist"!

3. "H. H." says I should not be so anxious to claim as mine the explanation I offered of the phenomenon to which reference has been made, since it is to be found in Biot, and in Lardner's "Mechanics." Now on this I remark, first, that it is bad logic. The conclusion is wholly unwarranted by the premises. If I claim the merit of a discovery, my claim will not be invalidated by showing that others have made the same discovery. By a reference to dates the question of priority may be determined; but this will go only a small way towards the determination of the question of originality. But, secondly, not only is "H. H.'s" conclusion fallacious; his premises are also unsound. I made no claim to originality in the matter. You, Mr. Editor, allowed Mr. Phillips to state his difficulty for the consideration of your readers, and I offered what seemed to me a satisfactory explanation of it, in which I showed that the phenomenon to which Mr. P. referred was a very obvious result of well-known principles. To have laid claim to original

*

I mean the principles of equilibrium, of which, by the way, "H. H." shows himself grossly ignorant, by supposing it possible (p. 275) that in the case of a body freely suspended, and at rest, the centre of gravity can be found anywhere but in the line passing through the point of suspension, and the centre of the earth.

discovery in such a case would have been supremely ridiculous, and akin to claiming the origination of the whole theory of statics; and I said nothing on the subject which any one less ingenious than "H. H." could, I think, have by possibility twisted into the assertion of such a claim.

"H. H." appears to have grounded his ridiculous charge on the fact, which I admit, that I called the explanation I offered, my explanation. I called it so to avoid circumlocution, just as "H. H.," at p. 454, calls the explanation he offered, his explanation. Then, I ask, are we to infer that "H. H." claims for himself the honour of being the sole and original discoverer of the notable fact, that if two weights be suspended by a cord over a pulley, and the pulley be made to revolve, for each revolution of the pulley, the portion of the cord on one side will receive an accession equal in length to the circumference of the pulley, and that on the other side an equal diminution? To this "H. H." ought to answer, 66 Certainly." Very well-I am not one of the few who will be disposed to contest his claim.

In conclusion, had "H. H." been more cautious in his choice of terms, he had not been reduced to the necessity of now admitting that an explanation which he characterized as an imagined one, is "perfectly correct;" or of entreating us to outrage our understandings by believing that what seems to us a downright and unqualified assertion, is nothing more than a mere supposition. Nor would he have blamed me for the employment of a form of speech, in which, without showing wherein his privilege consists, he freely indulges himself. The practical inference he ought to draw, therefore, is, to be a very great deal more cautious in time to come, than he has been in time past. I am, Sir, yours respectfully,

Hermes-street, Pentonville,
January 23, 1844.

PNEUMATIC COFFEE-POTS.

G.

Sir, Platow's Pneumatic Coffee-pot and Urn are justly held in estimation for the satisfactory manner in which coffee is made by them; but it is not to be denied that the former (Coffee-pot) is not so handy or convenient for ordinary use as could be wished. The moveable vase to be screwed on to the pot for the making of the coffee, and then to be screwed off for the pouring out the liquid by the same aperture, is not a very convenient arrangement, to say nothing of the awkwardness of making a screwed aperture

The Coffee-pot (fig. 1) is divided into two portions or compartments by a partition A, so placed, that the lower compartment shall be somewhat less than the upper, and capable of containing as much of the prepared fluid as may be wanted for use at a time. The centre of the partition is blocked into a well or dish, from the bottom of which an open tube descends to within an inch of the bottom of the pot. The well, or dished part, is covered by a wire gauze or perforated metal strainer of a convex form, as represented by the dotted curved line. The upper edge of the orifice communicating with the spout of the Coffee-pot, should not be more than half an inch from the bottom of the pot, i. e., half an inch lower than the extremity of the tube before mentioned; and the spout itself should be as high as the top of the Coffeepot. If now as much water be poured in as will fill the lower compartment, only so much of it will descend as will cover the extremity of the tube, the elasticity of the enclosed air preventing any more entering. This is practically of no consequence, as when the water is made to boil, steam is formed, the pres

sure of which causes the air to escape by the tube; and ultimately a vacuum is formed, which, on the pot being removed from the fire, becomes filled with the prepared fluid, being acted on by the pressure of the atmosphere. But a greater quantity of water, if desired, might in the first instance be made to enter the lower compartment, simply by inclining the Coffee-pot with the spout upwards, so as to make the dotted line a b, represent the level which the water would assume : the angular space above a b, would then represent the only portion occupied by air previous to the boiling of the water. It might be supposed that in the process of boiling, the fluid would be liable to be jerked out at the spout; but this would not be found to happen if it were conducted with ordinary moderation, and the ebullition prevented from being sudden and violent. The coffee grounds may be liable to settle on the strainer in a dense film, so as to intercept the pressure of the atmosphere and prevent the coffee flowing freely from the spout in pouring it out.

The application of a spoon would at once remove the obstruction, but as this might be considered troublesome, it