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ON ELECTRIC SPARK PHOTOGRAPHS: OR, only about half as fast as it does in air, and which w
PHOTOGRAPHY OF FLYING BULLETS, EC., not explode or even catch fire when an electric spark: BY THE LIGHT OF THE ELECTRIC SPARK.I made within it, or directly act injuriously upon tbt
photographic plate. The increased inclination of the II.
waves is very evident in Fig. 10. COING back now to the photographs, the next one These waves, revealed by photography, have a ver
U was taken with the view of illustrating the effect important effect on the flight of projectiles. Just as is on the inclination of the waves of the velocity of the the case of waves produced by the motion of a shig bullet. In this case the bullet was aluminium ; it was only which, as is well known, become enormously more ene: one-seventh the weight of the regulation bullet. In con- getic as the velocity increases, and which at high velocsequence of its lightness it travelled about half as fast ties produce as a matter of fact an effect of resistances again as the ordinary bullet (not N; times as fast as it the motion of the ship of far greater importance than the would have done if the pressure of the powder-gases had skin friction, so in the case of the air waves produced br been the same in the two cases), and in consequence of bullets ; in its flight the resistance which the bullet meer the higher speed the inclination of the waves is still with increases very rapidly when the velocity is raised greater than in the previous case. Further, in this beyond the point at which these waves begin to be case the bullet was made to pierce a piece of card formed. This being the case, I have thought it might be:shortly before it was photographed. The little pieces, teresting to see whether the analogy between the bebar that were cut out were driven forward at a high speed, iour of the two classes of waves might be even neare: but, being lighter than the bullet, they soon lost a large than has already appeared, and on turning to the beautifa
part of their velocity; they had in consequence lagged behind when they were photographed, but thoughtravelling more slowly (they were still going at more than 1100 feet a second) they yet made each its own air wave, which became less and less inclined as the bits lagged more and more behind; each, moreover, produced its own trail of vortices like that following the bullet. The well-known fact that moving things tend to take the position of greatest resistance, to avoid the effect of which the bullet has to be made to spin, is also illustrated in the photograph. The little pieces that are large enough to be clearly seen are moving broadside on, and not edgeways, as might be expected.
In order to illustrate the other fact that the angle of the waves also depends on the velocity of sound in the gas, I filled the box with a mixture of carbonic acid gas, and the vapour of ether, a mixture which is very dense, and through which sound in consequence travels
researches of Mr. Scott Russell, published in the Rept: of the British Association for the year 1844, in whic he gives a very full report on water waves and the properties, I found that he had made experiments De had given a diagram showing what happens when as tary wave meets a vertical wall. The wave, as would b expected, is, under ordinary conditions, reflected perfect making an angle of reflection equal to the angle of Ec dence, and the reflected and incident waves are alike : all respects. This continues to be the case as the an gets more and more nearly equal to a right angle, until the wave front, nearly perpendicular to the wall. along nearly parallel to it. It then at last ceases te reflected at all. The part of the wave near the instead gathers strength, it gets higher, it there ar travels faster, and so causes the wave near the wall to ahead of its proper position, producing a bend in the se front, and this goes on until at last the wave near wall becomes a breaker.
1 Lecture delivered at the Edinburgh meeting of the British Association by C. V. Boys, F.R.S. Continued from page 421.
In order to sce if anything similar happens in the CS
of air waves, I arranged the three reflecting surfaces of flector cut, growing up to a finite sphere about the end of sheet copper seen in Fig. 11, and photographed a maga- the reflector as a centre ; beyond this there are no more zine rifle bullet when it had got to the position seen. I centres of disturbance, the envelope of all the spheres Below the bullet two waves strike the reflector at a low projected upon the plate, that is, the photograph of the angle, and they are perfectly reflected, the dark and the reflected wave, is not therefore a straight line leaving off light lines changing places as they obviously ought to do. abruptly, but it curls round, as is very clearly shown, The left side of the V-shaped reflector was met at a dying gradually away to nothing. The same is the case, nearly grazing incidence ; there there is no reflection, but it is less marked, at the end of the direct wave near but, as is clear on the photograph, the wave near the the part that has been cut out. reflector is of greater intensity, it has bent itself ahead of The other point to which I would refer is the dark line its proper position as the water wave was found to do, but between the nose of the bullet and the wire placed to it cannot form a breaker, as there is no such thing in receive it. This is the feeble spark due to the discharge an air wave. The same photograph shows two other of the small condenser which clearly must have been on phenomena which are of interest. The stern wave has a the point of going off of its own accord. The feeble piece cut out of it by the lower reflector, and bent up at i spark precedes--or is to all intents and purposes simulthe same angle. Now if a wave was a mere advancing taneous with, it cannot follow—the main spark which
'hing the end of the bent-up piece would leave off ! makes the photograph. The feeble spark heated the air, suddenly, and the break in the direct wave would do the and the light from the main spark coming through this same. But according to the view of wave propagation line of heated air was dispersed, leaving a clear black put forward by Huygens, the wave at any epoque is the shadow on the plate. One spark casts a shadow of the resultant of all the disturbances which inay be considered other. Now it is evident that if the spark at the nose of to have started from all points of the wave front at any | the bullet had followed instead of having preceded the preceding epoque. The reflector, where it has cut this main spark by even so much as a three-hundred-millionth wave, may be considered as a series of points of disturb of a second, the time that light took to travel from one to ance arranged continuously in a line, each, however, the other, it would not have been able to cast a shadow. coming into operation just after the neighbour on one side We have the means of telling, therefore, which of two and just before the neighbour on the other. The reflected sparks actually took place first, or perhaps the order of wave is the envelope of a series of spheres beginning several, even though the difference of time is so minute. nith a point at the place where the wave and the re- Perhaps this method might be of some use in researches
now attracting so much interest in connection with the bullet has left the muzzle the imprisoned powder gase. propagation of electrical waves.
under enormous pressure, rush out, making a drauge On returning to the non-reflection of the air wave in past the bullet of the most tremendous intensity tending the upper part of the figure we have here, I imagine, obviously to drive it forward. While this draught dos optical evidence of what goes on in a whispering gallery. most assuredly hurry the bullet on its forward course, it The sound is probably not reflected at all, but runs does not tend to make it spin round any faster. Now í round almost on the surface of the wall from one part to the bullet were not hurried on at all after it left the another.
muzzle it would, travelling as in a screw of the same pitch We are now in a position to see how the reflection or all the way from the breach of the rifle up to the poir. non-reflection of air waves produced by a passing bullet, at which it is photographed, have turned round a certa. when they meet with some solid body, may produce a number of times which depend upon the distance travelled practical result which might be of importance in some and the pitch of the screw. If, however, the longitudica cases. Suppose a bullet to be passing near and parallel motion is hurried and the rotational is left unaltered the to a wall. Then if the velocity of the bullet and its pitch will be lengthened outside the barrel and the distance from the wall are such that the head wave meets rotation will have been less for any position than : the wall at an angle at which it can be reflected, especially, would have been if the bullet had not been accelcrated as in the case of Fig. II, if the reflected ray can only! in this way. If, therefore, we can find to what este return into the path of the bullet after it has gone, then the bullet has turned actually at the place at whic no influence whatever can be exerted upon the bullet it has been photographed, we can find the appare by its proximity to the wall. If, however, the head wave rotational lag and so working backwards get a measure would, if undisturbed, meet the wall at such an angle of the velocity acquired after leaving the muzzle. :3
that it could not be reflected, as for instance, in Fig. 12, order to accomplish this I drilled a series of Le when the head wave can be reflected by neither of the transversely through the bullet, each one at an angle: walls between which the bullet is passing, obviously the the previous one, the whole series being such that wave will become stronger and the resistance which it whatever extent the bullet had twisted, one at least, L. offers will, I imagine, become greater and if in this case the perhaps two, would allow the light of the spark to s upper plate be removed this extra resistance will be one- through it upon the photographic plate. Then from sided and must tend to deflect the bullet. This is quite photograph it is easy to see through which hole thels distinct from the well-known effect of a bayonet upon shone, and knowing in what position this was in the path of a bulletwhen a bayonet is fixed the rush breach, it is easy to find what fraction of balí at of powder gases between the bullet and the bayonet is over or above any whole number of half turns the bu quite sufficient to account for the deflection which every has twisted. Strictly the measure should be made. practised marksman allows for.
different distances to eliminate all uncertainty, but I have devised a method by which a problem of some only shot I have taken was sufficient to show that difficulty, about which authorities are, I believe, by no was a rotational lag equivalent, according to the meas means in accord, may be solved with a fair degree of made by Mr. Barton, to something under a two certainty. The problem is this, to find what proportion cent, acceleration outside the barrel. I do not att of the velocity of a bullet is given to it after it has lest any importance to this figure; the experiment was wir the barrel, or, what comes to the same thing, to find the with a view to see if the method was practicable and position in front of the barrel at which the speed is a it certainly is. I would recommend, where accura? maximum. The cause of this is evident. When the required, that having found as above about how much.
bullet has turned, that a second bullet should be drilled with a series of holes at about the corresponding position differing very slightly from one another in angular position, so that several would let the light through and thus give a more accurate measure of the rotation.
There is a point of interest to sportsinen which has given rise to a controversy which the spark photographs supply the means of settling. The action of the choke bore has been disputed, some having held that the shot are made to travel more compactly altogether, while others, while they admit that the shot are less scattered laterally, as may be proved by firing at a target, assert that they are spread out longitudinally, so that if this is the case the improved target pattern is no criterion of harder hitting, especially in the case of a bird flying rapidly across the direction of aim.
shot is filled with air waves of the greatest complexity. "They are not due to the cause already explained, but are, I believe, formed by the imperfect mixture of air with powder gases still accompanying the shot. The imperfect mixture of the two gases causes light to be deflected in its passage, thus producing striæ just as at the first mixing of whisky and water, striæ are seen (sometimes attributed to oil !), which disappear when the mixture is complete. I would mention, for the benefit of any one who may be tempted to continue these experiments, that a pair of wires such as are found to do so well when bullets have to be caught are not suitable, as one is sure to be shot away before such a bridge of shot is made between them as will allow a spark to pass. However, by using thick copper wires, one bent in the form of a screw, with the other along the axis, no such failure can occur and every shot that I have taken in this way
I was unfortunately not able, in the limited space and time that I have been able to employ, to take photographs of the shot at a reasonable distance from the gun, but I have taken comparative photographs at three or four yards only in which every shot is clearly defined, and in which it is even easy to see on the negative where the shot have been jaimed into one another and dented. The difference in the scattering at this short distance is not sufficient for the results to give any information beyond this, that shot are as easily photographed as bullets, and that no difficulty need be apprehended in attempting to solve any ques. tion of the kind by this method. The photograph, Fig. 13, represents the shot from the cylindrical or right-hand barrel. The velocity now is so low that individual waves are no longer formed by each shot. The whole space, however, occupied by the
has been successful. One can of course test the action of any material mixed with the shot. For instance, in one case I mixed a few drops of liquid oil with the shot and found them more widely scattered in consequence, not, as has been stated, held together by the oil as if they were in a wire cartridge. Of course, solid grease or fat may, and no doubt does, produce such a result, but liquid oil certainly does not.
And now I wish to conclude with a series of photographs which show how completely the method is under control, how information of a kind that might seem to be outside the reach of experiment may be obtained from the electric spark photograph, and how phenomena of an unexpected nature are liable to appear when making any new experiment. The result, however, is otherwise of but little interest or importance.
I thought I should like to watch the process of the
piercing of a glass plate by a bullet from the first shock step by step, until the bullet had at last emerged from
a photographic print and even, but less clearly, of the print in the text shows that these inclined air waves are inade
FIG. 14. the confusion it had created. In Fig. 14 the glass plate up of a series of dark and light lines at a very slig is seen edgeways just after the bullet has struck it. It is clear at once that the splash of glass dust backwards is already four or five times as rapid as the motion of the bullet forwards. A new air wave is just beginning to be created in front of the glass-coated head of the bullet and two highly-inclined waves, one on either side of the glass, reaching about three-quarters of the way to the edge, have sprung into existence. These are more clearly seen in the next figure; meanwhile it may be well to point out that the fragments of paper which are following the bullets have in this case-as the card was much nearer to the glass plate than in those previously takensome of them lost so much of their velocity and have in consequence lagged behind in a still higher proportion than the others, that they are travelling at less than 100 feet a second ; the more backward ones carry in consequence no air waves and there is no means of telling from the photograph that they are moving at all. In Fig. 15 the bullet has struggled about half way through the plate. The waves on either side of the plate have now reached the edge and are on their way back towards the centre again. They are caused in this way. When the bullet strikes the plate the violent shock produces a ripple or tremor in the glass which travels away radially in all directions, leaving the glass quiet behind. The rate at which this ripple travels may be found from the angle which these new air waves make with the plate, for taking any point on the plate and measuring up to the point where the air wave meets the plate and also the distance in air to the nearest point of the inclined air wave, we get two distances, the ratio of which is the ratio of the velocity of the disturbance in the
Fig. 15 glass to the velocity of sound in air. But much more than this is shown. An examination of the negatives or of inclination to the air wave itself, so that as we trait