Since the true cause of the explosive force of powder has been known, some unaccountable errors have been committed in attempting to solve the question. Some philosophers, forgetting themselves, have imagined that the effect arose from the generated fire or flame, as they considered it, acting on the vent or choke ; an action which, it is very evident, would tend only to burst the rocket, not to make it fly. But we may pass by these errors, since there is no difficulty whatever in the question. As the powder or the composition, which does not differ very materially from that, is condensed to one half the space which it occupies when in a gun, the production of air, proportioned to the hulk of this composition, is nearly double what it is from gunpow der in a charge. If taking according to the relative weight, it approaches nearer to that ; but is still in ferior, on account of the greater disproportion of the nitre necessary to insure a fire sufficiently slow, whence the whole of the charcoal is not converted into gas. But as it is not necessary to be very accurate for the present purpose, we may consider that the rocket com position produces 500 times its bulk of gas at the mean temperature of the air, while its elastic force is increas ed by the heat to not less than 2000. Thus the rocket of three inches diameter and too feet long, (to assume a case,) tvill furnish 96.000 cubic inches of gas of the same density as the atmospheric air, at the same tem perature, or at the elevated one at which it is actually generated, 384,000.
In the rocket, now, of the assumed dimensions, the whole of this fluid must be discharged within the space of 20 seconds from an aperture of 1 inch in diameter ; and this, therefore, requires an issuing ve locity for the stream of air, (or flame as it is called,) equal to that which would result from 19.200 cubic inches flowing in a stream of that diameter through every second of time. Here, then, is the source of the mot ing power. which becomes a case of recoil precisely analogous to that which takes place when a piece of ordnance is fired. The resisting body in this case, as in that of a piece of ordnance fired without a wad or a shot, as is done in Dr. Hutton's pendulum, is the air ; and the flight of the rocket thus depends on a compari son between the weight to be moved, (adding to it the effect of gravity and the anterior resistance of the atmos phere,) and that of the resistance which the air opposes to the issuing current.
We have here assumed grounds of computation which are not exact, but quite sufficient to explain the gene rat principle. But it is plain, that, with accurate data, such as the exact quantity of composition contained in the rocket, the exact quantity of gas which a given weight of it would produce, and measure of the temperature, the problem that relates to the velocity of a rocket at any one point of time, and so on for the whole flight, might be calculated Dr Hutton's ele ments of calculation for the initial force of gunpowder might, with the necessary addition, be applied to the solution ; if not with accurate results, when the intri cacy of all the circumstances are considered, yet with not much less than in the case of the firing of ord nance, and the quantity, whether 01 the recoil or the pro pelling power. But our readers would not thank us to
enter into these calculations with the requisite accuracy, • even if our limits permitted us to enlarge on this part of our subject.
We shall, therefore, draw in on this question, and only further say, that in considering the natut e and action of this force, it might probably he made use of to advantage in obtaining that end of which we spoke before, namely, overcoming the inertia of rock ets. To effect this purpose, a solid obstacle or plate affixed to the frame might he opposed to the issuing current of air, which, by offering a steady and firm re sistance to it, would produce a far greater initial effect than can be obtained from that of the atmosphere merely.
From this explanation of the action by which a rocket is caused to fly, the reason for its very peculiar construction becomes evident. If there were no bore, no part of it could at any time be inflamed greater than an area of the cylinder; and the air produced from so small a surface would be insufficient to municate the requisite velocity. In consequence of the sizo or length of the bore, nearly the whole length of the composition is inflamed at the same time, and hence a rapid issue of air is the consequence. Hence also we can understand why the velocity of a rocket tends to increase during every successive part of its flight; because as the diameter of the bore becomes enlarged by burning, a much greater extent of sur face is occupied in the production of gas. Hence also the cause of bursting becomes evident ; as in cases of fracture of the composition, so large an extent of it may take fire, that the air cannot find a sufficiently free issue at the vent. The necessity of choking or of contracting the vent hole, must now also be so evident that we need take no notice of it.
It remains yet to inquire respecting the use of the stick, and the deviations to which the flight of a roc ket is subject. If we assume the simplest case, or that of a vertical rocket, it is plain that it is subject, in a still atmosphere, to two counteracting forces only ; namely, that of gravity, and that of the anterior re sistance of the atmosphere. If therefore it could be retained in the vertical line without a stick, or any other appendage, it would ascend in a perpendicular direction. But it is subject to several disturbing forces which render it impossible for it to preserve that line. One of these is the wind ; another arises from its own irregularity of form, which prevents it from meeting with an equable resistance on all sides from the cur rent of air which it makes by its own flight ; and the other and principle one is the perpetual variation of its centre of gravity which arises from the burning out of the composition.