If now a stick be attached to the lower extremity, and of sufficient length, parallel also to the axis, it is plain that at whatever instant it tends to deviate from the line of its flight, the rapid stream of air which is passing along parallel to the stick, will meet it on the opposite side and restore the original position. The case is precisely that of a rudder in a ship, si milarly acted on by the current of water that runs along the keel, in such a manner as to produce the effect of restoring the true position wherever it may deviate ; and further, in this particular case, of pro ducing a new enc. tv the rudder to meet the current. The length of the stick is also necessary, partly to enable it to act with the greater effect on the passing current by its distance from the centre of gravity of the rocket, and partly to enable it the more easily to counteract or compensate that change in this centre which arises from the burn ing out of the composition.
In flights at low elevations, and at angles less than the perpendicular, the same reasoning, with some va riations, is applicable. These are, however, so obvious, that we need not enter on them. We must, however, remark, that the weight of the stick, in these cases, produces an effect which does not occur in the verti cal flights. Its tendency is to make the rocket assume an angle of elevation greater than that at which it is placed on the frame. Thus, to produces a curve calcu lated for an initial direction of 45°, the rocket will re quire an elevation which may be only or even less. The explanation of this is easy. At the moment the rocket quits the support, its velocity is yet small ; or the steerage way, to use a sea phrase, is insufficient to allow the rudder its full action. The weight of the after end of the stick thus causes it to droop, and consequently to elevate the rocket so as to increase the angle with the horizon. Hence the singular curves which are assumed by such rockets at the com mencement of their flights.
From the action of the stick we can also account for the effects of wind in disturbing the flight of a rocket, and in altering the line of its direction ; but we need not dwell on matters so obvious. But it is plain, that from the same great power which it thus possesses, it is most important, that in quitting the support, it should run clear : as the slightest impedi ment or irregularity that may affect its extremity, in particular, may produce very considerable deviations. It has been proposed to steady the flight of rockets by means of wings, so disposed, on opposite sides of the case, as to form the whole into the shape of an equi lateral triangle. But this method is not efficacious,
neither is it attended with any particular advantages in other respects. In the books it is also said that rockets may be steadied by a bullet attached to the end of a string at their extremity, which is palpable nonsense.
Of Line Rockets.
These are so exactly like sky rockets in their con struction, that the same rules for filling the cases, and for the composition, are applicable to both. But as they are seen at a less distance, they admit of a variety of fire, which, in the sky rocket, is almost superfluous. This is the brilliant iron fire ; and the artist may adopt, for this purpose, any of the strong composi tions under the head of brilliant fires, or else that sky rocket composition described in the table, for those into which iron enters. For the reader's convenience we may here give it again.
Saltpetre 8.
Sulphur 4.
Mealed powder 2.
Pounded iron or steel filings 1# part.
If lines can be stretched far enough, there need be no limit, as in sky rockets to the length of the corn-. position ; but in no case is it necessary that the com position should burn longer than the line will admit, as it would at the end of its range remain stationary. Thus. SC we have just shown that the average flight of a sky-rocket of one pound is 480 yards, 500 will be a sufficient length for the line. If it can be carried farther, an additional number of diameters of compo sition may be left above the bore in filling it ; and it is easy by trial to compute the exact relative lengths of the line, and the nature of the composition, that this may be finished at the end of the range. We may also here remark, that the flight of a line rock et may be too rapid for the eye to follow it con veniently, particularly if it is a simple one, as it has very little resistance to encounter. In such a case it will be proper to reduce the length of the bore, which may easily be bone by using a shorter spindle. But if the line rocket is to be compounded, or to carry any considerable weight, the artist may adhere to the usual proportion for skyrockets. In all cases it is easy to understand that the flight may be retarded by diminishing the length of the bore, or by increasing relatively the unbored part of the composition, in which case the effect is more pleasing than when the velocity is very great, but it is useless to give any positive rule on this subject, as the artist must be regulated by the length to which lie can stretch his line.