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j;—{— The fresh water (360 gallons Compositions for rockets of various stzes. The general composition for rackets is, Saltpetre 4 |b, Sulphur 1 lb. Charcoal 1 Ib. But for large rockets, Saltpetre 4 |b. Sulphur 1 Ib. Meal powder 1 Ib. For rockets of a middle size, Saltpetre> 3 Ib. Sulphur 2 |b. Meal powder 1 1b, Charcoal 1 Ib. When rockets are intended to mount upwards, they have a long slender rod fixed is the lower end, to direct their motion. Rockets have often been employed to throw among an enemy’s army, by firing them at oblique angles. In this way they have been used by the Indians for many ages; and they have recently been introduced into the English service by Mr, Hume and colonel Congreve. Theory of the flight of rockets.—Mariotte takes the rise of rockets to be owing to the im- ulse or resistance of the air against the flame. eerie accounts for it thus. Conceive the rocket to have no vent at the choke, and to be set on fire in the conical bore; the consequence would be, either that the rocket would burst in the weakest place, or that, if all parts were equally strong, and able to sustain the impulse of the flame, the rocket would burn out immoveable. Now, as the force of the flame is equable, suppose its action downwards, or that upwards, sufficient to lift 40 pounds; as these forces are equal, but their directions contrary, they will destroy each other’s action. Imagine then the rocket opened at the choke; by this means the action of the fame downwards is taken away, and there remains a force equal to 40 pounds acting upwards, to carry up the rocket, and the stick or rod it is tied to. Accordingly we find that if the composition of the rocket be very weak, so as not to give an impulse greater than the weight of the rocket and stick, it does not rise at all; or if the composition be slow, so that a small part of it only kindles at first, the rocket will not rise. The stick serves to keep it perpendicular; for if the rocket should begin to tumble, moving round a point in the choke, as being the common centre of gravity of rocket and stick, there would. be so much friction against the air, by the stick between the centre and the point, and the point would beat against the air with so much velocity, that the reaction of the medium would restore it to its perpendicularity. When the composition is burnt out, and the impulse upwards has ceased, the common centre of gravity is brought lower towards the middle of the stick; by which means the velocity of the point of the stick is decreased, and that of the point of the rocket is increased; so that the whole will tumble down, with the rocket end foremost. All the while the rocket burns, the common centre of gravity is shifting and getting downwards, and still the faster and the lower as the stick is lighter; so that it sometimes begins to tumble before it is quite burnt out: but when the stick is too heavy, the common centre of gravity will not get so low, but that we rocket will rise straight, though not so ast. From the experiments of Mr. Robins, and other gentlemen, it appears that the rockets of 2, 3, or 4 inches diameter, rise the highest; and they found them rise to all heights in the air, from 400 to 1254 yards, which is about three quarters of a mile. See Robins’s Tracts, vol. 2, p. 317; and the Philos. Trans. vol. 46, p-.578. The theory of rockets, however, is stil} in a very imperfect state. Mr. Moore of the Royal Military Academy, Woolwich, has published some e ays the subject in Nicholson’s Journal; but we still need a complete treatise on this interesting and rather abstruse subject. 293