PERHAPS the chief drawback to a sufficiency of life preserving articles being provided on board our packet steamers and other passenger ships, has been the unfitness for "other purposes of any such articles commonly in use. Thus it may have often occurred to a shipowner on thinking over his responsibilities, which no doubt shipowners as well as other folks sometimes do, that, when providing a vehicle for the conveyance of perhaps hundreds of human beings across the sea with its attendant dangers, he is morally bound to provide them with every procurable means for insuring their safety in the event of any of those casualties happening, from fire, storm, or collision, which are of everyday occurrence.

The results of his thinking may be, that before launching forth his craft with her precious freight on board, and bidding her " God-speed," he will ascertain that she is sound and in good repair, and that her rigging, sails, anchors, boats, and other stores, are all complete and trustworthy. He may then provide for her an able and sufficient crew, and place all under the charge of a skilful captain. He may further cause one or more of her boats to be fitted as lifeboats.

The thought may, however, then occur to him that after all, in the event of a casualty, the boats may be injured or upset, or that they cannot be got out, and that therefore in such an emergency, some of the well-known contrivances of life-buoys or life-belts for the separate support of individuals in the water might afford additional security to their lives. He at once decides on having two or three, or it may be half a dozen for the purpose of throwing to any persons accidentally falling overboard, or getting upset in a boat alongside; but he perceives that to have them in numbers at all approximating to the number of individuals on board, would be attended with serious inconvenience, in addition to their expense ; as they would occupy much room in a closely-packed ship, where every portion of the space is valuable. This inconvenience accordingly turns the scale; he consoles himself with the reflection that he has already done much more than many others do ; that few ships are so well provided in all the above respects as his, and comforting himself with these reflections, he feels satisfied that he has done his duty to those who have intrusted their lives to him, and, after insuring his ship, he sends her on her way, trusting that, with her wonted good fortune, she may have need for none of these things.

We are happy, however, to know that shipowners may in future be relieved of this difficulty, and of the anxiety which it no doubt has sometimes occasioned them, since a description of life-buoy has been invented by an American engineer, a Mr. NATHANIEL THOMPSON, of New York, which fulfils the two purposes of an ordinary seat and of a life-buoy.

Now as all vessels must have seats for those on board them, and such seats must take up room, the difficulty of stowage is at once got over, and there seems no reason why there should not be on board any ship as many life-preserving seats as there are persons in her.

We have seen experiments made with Mr. THOMPSON'S invention, and as it forms at once an efficient life-buoy, and a useful and ornamental seat, we do not hesitate to pronounce him as entitled to much credit for his ingenuity, or to recommend his invention for general adoption.

The following is a brief description of the life-preserving seat, which, however, it is difficult to convey a distinct idea of without a diagram.

Its general form, when in ordinary use, is nearly that of a common four-legged stool, or a chair with the back cut off; the upper portion of it, or the seat itself, as distinguished from the legs, possesses the buoyant power which is contained in two metallic air-tight chambers incased in wood. It is divided vertically down the middle, and so enabled to open and shut, being confined in either position by metal clumps, which slide up and down; when closed or shut up, it is a seat; when open, it is a life-buoy; the two air-chambers being then extended horizontally, connected at the sides, and allowing sufficient space for a stout person to stow between them, in which position he may float very much at ease, having the free use of his arms, with which and his feet he can readily propel himself through the water, the one chamber floating in front, and the other behind him.

Its dimensions as a seat are about 18 inches high, and 16 inches square on the top; when extended as a life-buoy it is about 3 feet long, and 2 feet wide.

This seat is, we understand, already in extensive use on board American passenger ships, as we have no doubt it will also soon find its way into ours.