IT will be remembered that some months ago there occurred a lamentable boat accident in the River Thames, when no less than ten promising youths, cadets on board a training-ship, were drowned. The boat was under sail at the time, and was said to be a " good and safe boat," but she had no ballast.

We do not revert to this melancholy accident with a desire to impute blame to any person in charge of the cadets on board the boat in question, or to awaken any sad recollections; but because it may be considered from the evidence then given at the coroner's inquest, that, whilst the palpable cause of this accident was the want of ballast in the boat, most mistaken notions, even amongst practical seamen, exist on the subject; and that from such mistaken •views future accidents -may be expected to occur.

At the inquest no less than three witnesses stated that they did not approve of carrying ballast in boats. One of these witnesses was the captain of the ship, another was her boatswain, and the third was a Greenwich waterman. They were all, therefore, practical men. The captain, indeed, went so far as to say that " many lives had been lost by placing ballast in boats." As, on the other hand, we are persuaded that many more lives have been lost by the upsetting of boats under sail, owing to their having no ballast to counteract the pressure of the wind on their sails, or to their being insufficiently ballasted, it may be useful to devote some consideration to the subject.

The phenomena of upsetting may be thus plainly described. Boats, in common with other floating bodies, have a central axis, or centre of motion, round which they have a tendency to revolve; which centre will be higher or lower with reference to the general mass of the body or structure, according to its shape and to the distribution of weight within the same; whilst the stability or resistance to upsetting will be great or little according to the relative positions of this centre of motion, and the centre of weight called the centre of gravity.

Thus the lower the centre of gravity is below the centre of motion, the greater is the stability. When the two exactly correspond, there is no stability, but a tendency to revolve round the common centre; and when the centre o'f gravity is carried above the centre of motion, what is called in mechanics an unstable equilibrium is produced, or, in plain English, the body is topheavy, and must upset. The first of these axioms may be illustrated by the common use of metal keels, or of ballast stowed in the lowest part within a vessel or a boat.

The second, by an empty cask which has no stability, but a tendency to revolve round the common centre; whilst the third has been too often fatally illustrated by persons climbing the masts of small boats, and thus upsetting them.

It follows, therefore, that the addition of any weight placed low in a boat or other vessel, as ballast, must add to her stability, and thereby make her less likely to upset.

But, no doubt, all this would be admitted by the three witnesses above referred to, and their disapproval of ballast in boats must have arisen solely from the fact of its causing a boat to sink after upsetting, instead of still floating, although entirely immersed; and their choice therefore must have been, of what they thought a lesser danger in preference to a greater. If, however, we can make it clear that, by the selection of a suitable material for ballast, both dangers may be avoided, we shall render a service —and this can be readily done.

Now it so happens that a most mistaken notion regarding ballast very commonly exists, many persons supposing it to be necessary that it should be composed of lead, iron, stone, or some other substance heavier than water. There could not, however, be a greater mistake, since equal weights of any substance afford similar ballast; the only difference being that the heavier the substance the more concentrated it will be, and vice versa; so that in cargo vessels, in which the whole of the interior space is of much value, the heavier description of ballast is more suitable, because the required amount will occupy less space.

Indeed, as any ordinary decked cargo-vessel would sink if filled with wafer, whether ballasted with metal or not, it would be of no advantage in such vessels to use a lighter description of ballast. The case as regards boats and small pleasure-vessels is, however, quite different; and for a long period it has been the custom in the ships of the Royal Navy to ballast their boats with small casks, or barrels of water, which would float of themselves if immersed, and would therefore have no tendency to sink a swamped boat.

But if a boat is ballasted with any substance lighter than water, should she get swamped or upset, the whole of the surplus buoyancy of the material would, if it were properly secured, be transferred to the boat herself, and therefore help to float her. Accordingly, in the life-boats of the x NATIONAL LIFEBOAT INSTITUTION, nearly half the ballast is composed of cork in water-tight boxes stowed under the deck, which is also watertight.

As long, therefore, as. the boat is tight and sound these boxes of cork act solely as ballast, but should she get stove in below the deck, the surplus buoyancy of the cork would then prevent her sinking too deep to be manageable in a high sea.

As, however, this would be too expensive a description of ballast for ordinary boats, and from its great lightness would take up too much room, a more suitable material would be wood, and a light or heavy description of wood could be adopted as might in each case be most convenient.

Thus fir wood has about.half the specific gravity of water, weighing about half as much, bulk for bulk, and therefore every cvvt. of fir ballast in a boat would impart about 56,lbs. of surplus buoyancy to her.

In those cases, however, where a sufficient quantity of fir ballast would occupy so much space in a boat as to be inconvenient, a heavier description of wood might be employed. The most convenient shape in which to employ it would perhaps be in two balks, or logs, placed side by side, fore and aft, above the keelson, and lashed securely to the bottom of the boat; but the same rule might not apply to all boats, as the position which would be convenient in one might be inconvenient in another.

We strongly.recommend to all owners of sailing-boats, and especially of open pleasureboats, the serious consideration of the subject, believing, as we do, that the general adoption of wooden ballast would be the means of preventing many accidents, and of saving many human lives.