Dr. Desaguliers illustrates his hypothesis in the following manner:—Conceive the Rocket to have no vent at the choak, 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 forty pounds; as these forces are equal, but their directions contrary, they will destroy each other’s action.
Imagine then the Rocket opened at the choak; by this mean the action of the flame downwards is taken away, and there remains a force equal to forty 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.
To this we shall add the late Doctor Hutton’s philosophy, on the ascent of Rockets; who says, that at the moment when the powder begins to inflame, its expansion produces a torrent of elastic fluid, which acts in every direction; that is, against the air which escapes from the cartridge, and against the upper part of the Rocket; but the resistance of the air is more considerable than the weight of the Rocket, on account of the extreme rapidity with which the elastic fluid issues through the neck of the Rocket to throw itself downwards, and therefore the Rocket ascends by the excess of one of these forces over the other.
This, however, would not be the case, unless the Rocket was pierced to a certain depth. A sufficient quantity of elastic fluid would not be produced; for the composition would inflame only in circular coats, of a diameter equal to that of the Rocket; and experience shows that this is not sufficient. Recourse then is had to the very ingenious idea of piercing the Rocket in a conical hole, which makes the composition burn in conical strata, which have much greater surface, and produce a much greater quantity of inflamed matter and fluid. This expedient was certainly not the work of a moment.
The stick serves to keep it perpendicular; for if the Rocket should begin to tumble, moving round a point in the choak, 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 at the point of the Rocket is increased; so that the whole will fall down, with the Rocket end foremost.
During the time the Rocket burns, the common centre of gravity is shifting and getting downwards, and still faster and 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 of the Rocket will rise straight, though not so fast.
From the experiments of Mr. Robins, and other Gentlemen, it was found that the Rockets of two, three, or four inches diameter, rise the highest; and they are found to rise to all heights in the air, from 400 to 1,254 yards, which is about three-quarters of a mile. For further particulars respecting the theory of the flight of Rockets our readers are referred to Robins’s Tracts, vol. 2.—Philosophical Transactions, vol. 46, page 578: and more particularly to Mr. W. Moor’s “Treatise on the motion of Rockets,” in which they will find the subject elegantly treated.