The last project for varying the effects of sky-roc kets which we shall mention, is that of causing them to suspend a white light in the air after they are burnt out. It requires some little care to make the requisite ma chinery, but it is neither expensive nor difficult. For this purpose a parachute is attached to the rocket, which is so combined as to open when it is expended, and thus to check the descent of the case and stick The parachute for this purpose may be applied in different ways ; but it is perhaps most conveniently done in the fol lowing manner: being made of four or more pieces of slender whalebone, with a thin piece of linen stretched over them, it is attached to the lower end of the rocket just at the choke, so that its open end may lie upwards. Thus when the head of the rocket is turned downwards the parachute will act ; and care should be taken that it does act freely, and without turning inside out before any attempt is made to complete the arrangement. The rocket head is then filled with a white or blue light, in the manner described before, with a priming hole com municating with the top of the composition. A hole is also made in a lateral direction, for the carrying a leader to disengage the parachute at the mo ment the rocket is burnt out. The parachute must then be collected neatly round the rocket with the points upwards, which are to pass beyond the head, and to be enclosed in a light paper cone that may easily he disengaged. To insure that, the cone is made in two parts, and secured by a bit of cotton quick-match, which is to be also conducted in a spiral manner round the rocket and parachute together, so as to render the whole as compact as possible, after which the whole is to be covered with a single fold of weak paper. This quick-match is connected with the leader from the roc ket head, and with that which serves to set the light on fire ; but it is necessary so to arrange these two leaders, that the parachute may disengage itself before the light takes fire. As soon as this match is burnt, the para chute is disengaged ; an effect which may be aided by forcing the whalebone ribs together, as is easily com prehended ; and as the weight of the rocket cases causes it to overset as soon as the composition is burnt out, the parachute is then brought into action, so as to suspend the light, which thus continues to burn as it descends slowly. We need scarcely add, that many minute atten tions are required to make this act well. but that a dex trous mechanic can find no difficulty in it.
The parachute may also be attached to the middle or end of the stick ; but in this case great care must be taken that the leader which is to disengage it may no take fire by the burning of the rocket, until it receives its fire from the cod of the composition. For this put. price- it must be made strong where it passes below the rocket, and is exposed to the stream of fire.
On the height to which Rockets ascend.
In Mr. Robins's trials, the pound rockets were found to ascend perpendicularly, from 450 to 500 yards; and from some measurements made in the flights of those that were used at the great firework in the Green Park, they were found to range from 440 to 526, in general ; although there were some that rose to 615 yards. Mr. Robins also seemed to think, in his first experiments, that all sizes of rockets had pretty nearly the same flight. This, however, is not the fact. The ratio of increase indeed is not regular, nor does the ascent correspond in any way to the magnitude of the rocket ; but the greatest ascents, or the longest ranges, if fired at an angle of 43°, are ob tained with the larger sizes. The time for these ascents varies from ten to fourteen seconds. That time
does not bear a regular ratio to the altitude of the ascent ; because a considerable part of the force of the recoil is lost in overcoming the inertia of the rocket. That time, of course, is the same for all ; so that those which have the power of flying highest, increase their ranges, as these regard the time of flight in a certain progressive ratio.
Taking Mr. Robins's computation of 600 yards for the limit of ascent in signal rockets, that gives an elevation of one-third of a mile nearly. Hence if the light of the exploded stars which are the proper ornaments of a sig nal rocket, is sufficiently strong, and the atmosphere is clear, such a signal may be seen at a horizontal dis tance of fifty miles at sea, or in a level country where there are no advantages Its rise above the horizon is sufficient to render it quite visible. Mr. Robins need not have doubted if the light was visible at such dis tances ; as it has been ascertained that the common blue antimonial light, which burns from a case only an inch in diameter, may be seen at the distance of seventy miles without difficulty, and probably much more. In this the mass of light is considerably less than that produced by the stars of a signal rocket, and the composition is, as nearly as possible, the same in both.
In a subsequent set of experiments made by Mr. Canton and Mr. Robins together, rockets of two inches and a half in diameter were adopted. Some of these rose to 500, some to 600, and one to 690 yards ; and here, contrary to Mr. Robins's former opinion, it was invariably found that the largest rose the highest. Some larger rockets were afterwards made by Mr. Da Costa, of about three inches and a half in diameter ; and in these, the vertical ascents were 833 and 915 yards. Another trial, made with one of four inches in diame ter, gave a perpendicular flight of 1190 yards. The last of these experiments was made in April 1750; and on that occasion there were twenty-eight rockets fired, made by different persons and of different sizes, varying from the diameter of an inch and a half to one of four inches. The most remarkable flights of these are here tabulated, for the convenience of in spection.
In these experiments, the four inch rockets failed, having overset in their ascent ; probably from negli gence in the stick, or too great a length of charge.
At the same time, trials were made on the largest sizes of rockets which, as far as we know were ever constructed. These were twenty-four inches in dia meter; one of them rose to 784, and another to 833 yards. It is evident that these are both failures ; and we have no doubt they are to be attributed to defects in the construction. It is not possible to use exactly the same proportions, either in the length of the case, the proportional length of the bore, or the nature of the composition, in large as in small sizes. If Mr. Banks's and Mr. Da Costa's largest rockets were formed on the scale of the one pounders, it was impossible that they should have succeeded. In such cases also, the stick, the cap, and all the minutes parts of the rocket require a degree of care to obtain the best results, which are of little comparative moment in the smaller calibers. Nor is it possible that rockets of such enor moos diameters could have been driven properly by hand, as it is probable these were ; the pile-engine not having been generally introduced till it was applied Sir William Congreve to his military rockets. Thus, then, the failure of these large machines may easily be accounted lor, without assuming, as these experimenters or their reporters have done, that the sizes of rockets could not be augmented with advantage.