Recently a chronographe électro-balistique has been invented by M. P. le Boulengé, Lieutenant d'Artillerie, which has been tried and compared with Major Navez's electro-balistic pendulum, by M. Melsens, Membre de l'Académie Royale de Belgique. He states that M. Boulengé, in his memoir, renders what is just to Major Navez, remarking, "Cet officier en effet a donné, le premier, à l'Artillerie un appareil remplissant toutes les conditions exigées pour être utilement employé aux recherches balistiques; on sait que les essais faits en Angleterre, en France, en Allemagne par des savants illustres, par des militaires distingués, aidés d'ingénieurs et de fabricants habiles, avaient laissé sans solution pratique la question de la détermination des vitesses des projectiles pour tous les cas possibles, tant pour les armes portatives que pour les armes de fort calibre, tirant sous des angles variables."1 Experiments were made with various powders to compare the two Belgian instruments. The velocity of the same projectile was measured by both instruments for a point 35 metres from the gun. The following list gives the mean velocities in m.s. of five or six rounds for each kind of powder as measured by the two instruments:2

Or, converting the differences of the means of the velocities, as determined by the two instruments, into feet-seconds, we find them: -19·7,—31·2, -46,-13.8,-18.7, -17.7,-12.5,-10.5, -22.0, -14.8,-11.2, 0.0 and +6.6. M. Melsens appears to give the preference to M. Boulenge's instrument, but he makes the remark, "Des expériences novelles et nombreuses diront où est la vérité absolue."3. Now it is really remarkable that two instruments with any pretensions to accuracy should give such contradictory results. Five or six shot are fired, the velocity in each case is measured by two instruments. The mean of the velocities given by one instrument is compared with the mean of the velocities given by the other, and they differ in one case 31.2 f.s., in another 22.0 f.s., in another 19.7 f.s., and so on. The difference of the means varies from -31.2 f.s. to + 6.6 f.s. Thus so long as instruments are used without check, there will always be uncertainties to be settled by new and more numerous experiments. It is the great fault of these instruments that, whilst they are something near the truth, they are not sufficiently exact to give the law of the resistance of the air. But this is the problem that must be solved by any instrument that pretends to accuracy. This must be regarded as the test of the value of any chronograph. Robins was not content merely to determine a list of initial velocities, for he at once proceeded to deduce the resistance of the air to projectiles moving with any

1 Melsens, Rapport sur un Chron., 1864. p. 5.
3 Melsens, p. 24.

2 Ibid. Rapport, p. 21.

velocity. Hutton did the same. Turn to Didion's Treatise, and there it is found that Chapter I. is devoted to the consideration of projectiles moving in a vacuum, but in Chapter II., when he comes to the consideration of a practical case, he is met by the old difficulty of the amount and law of the resistance of the air. He also requires to know in what way, and to what extent, the resistance of the air is affected by the form and size of the moving body. Now it is remarkable that the electro-ballistic pendulum should have been in such general use for more than ten years, and that no satisfactory attempt should have been made to confirm, or otherwise, the laws determined by the old ballistic pendulum for round shot, and to extend them to cylindrical bolts with variously-formed heads. So far as the comparison of the two pendulums is known to have been attempted, we have seen, on the authority of M. Didion, that they give a different law, and that he considers the results of the old ballistic pendulum the more satisfactory of the two. And I am the more inclined to believe that nothing had been attempted with success up to last year from the remarks made by Major Navez in his "Considérations sur les Expériences de Balistique en ce qui concerne la Mesure du Temps," 1865. In that tract, he explains in general terms the method to be pursued in order to determine the resistance of the air. He recommends that the screens should be placed as much as 100 metres apart. He finds that M. Boulenge's chronograph cannot measure an interval of time exceeding 0"-1; that of Colonel Leurs cannot exceed 0.227964. He thinks it would be advantageous to have a superior limit of 0"-32, but contents himself with 0"-2. Major Navez remarks in his introduction: "Nous avons souvent entendu, nous entendons encore chaque jour, des officiers d'artillerie regretter que l'on ne connaisse pas, d'une manière plus précise, la véritable expression de la résistance de l'air quant à sa forme analytique et à sa valeur." And again, "Jusqu'à présent on connaît peu de chose sur la loi de la résistance que l'air oppose aux projectiles allongés; plus tard, lorsque des expériences complètes auront fait découvrir la véritable loi de cette résistance, on trouvera probablement le procédé d'interpolation par arc de cercle bien imparfait. C'est pourquoi nous tenons à établir qu'il s'agit seulement ici d'approximations, grossières peut-être, mais suffisamment approchées de la vérité pour éclairer nos investigations." And the following remark is worthy of particular attention: "Quand on examine en masse les résultats obtenus dans les expériences sur les vitesses conservées par nos projectiles allongés, à différentes distances de tir, on doit se trouver fort satisfait, en ce sens, que de ces résultats ressort, à l'évidence, la bonne conservation de la force vive. Mais si on se livre à un examen plus minutieux, on voit que ces résultats ne sont pas, en général, assez réguliers pour qu'on puisse les employer à établir la loi de la résistance que l'air oppose aux nouveaux projectiles." The perusal of the "Considérations may be strongly recommended to any one who wishes to understand the true state of the case and the difficulties that have to be overcome. I have always held, and still hold, that as nothing really good has been effected by the ordinary chronographs, so nothing can be done by them. Rather than attempt to

1 Considérations, p. 4.

2 Ibid. p. 17.

* Ibid. p. 34.

improve upon old methods, I decided to apply a known method of experimenting which was new to this branch of science.

After a due consideration of all circumstances of the case, it appeared that the following conditions must be satisfied by a chronograph worthy of perfect confidence :

(1) The time to be measured by a clock going uniformly.

(2) The instrument to be capable of measuring the times occupied by a cannon ball in passing over at least nine successive equal spaces.

(3) The instrument to be capable of measuring the longest known time of flight of a shot or shell.

(4) Every beat of the clock to be recorded by the breaking of the same galvanic current, and under precisely the same conditions.

(5) The time of passing each screen to be recorded by the momentary interruption of a second galvanic current, and under precisely the same conditions.

(6) Provision to be made for keeping the strings or wires of the screens in a uniform state of tension, notwithstanding the force of the wind and the blast accompanying the ball.

To gain 'assistance in carrying out the above conditions practically, I consulted a great variety of books on chronographs, electric telegraphs, clock making, &c. A description of one of the most recently constructed chronographs for an Observatory will be found in C. A. F. Peter's Ueber die Bestimmung des Langen unterschiedes zwischen Altona und Schwerin. In Loomis's Practical Astronomy (New York, 1855), it is stated that this method of recording transits had been employed exclusively at Washington Observatory since 1849, and allusion is made to the conical pendulum then (1855) in use at Greenwich to regulate the velocity of rotation of the cylinder of a similar instrument.

The following is a description of the chronograph as constructed, and of various useful appendages. Fig. 1 gives a general view of the chronograph. A is a fly-wheel capable of revolving about a vertical axis, and carrying with it the cylinder K, which is covered with prepared paper for the reception of the clock and screen records. The length of the cylinder is 12 or 14 inches, and the diameter 4 inches. B is a toothed-wheel which gears with the wheelwork M so as to allow the string CD to be slowly unwrapped from its drum. The other end of CD being attached to the platform S allows it to descend slowly along the slide L, about inch for each revolution of the cylinder. E, E are electro-magnets; d, d' are frames supporting the keepers; and f, f' are the ends of the springs which act against the attraction of the electro-magnets. When the current is interrupted in one circuit, as E, the magnetism of the electro-magnet is destroyed, the spring f carries back the keeper, which by means of the arm a gives a blow to the lever b. Thus the

marker m is made to depart from the uniform spiral it was describing. When the current is restored the keeper is attracted, and thus the marker m is brought back, which continues to trace its spiral as if nothing had happened. E' is connected with the clock, and its marker m' records the seconds.

E is connected with the screens, and records the passage of the shot through the screens. By comparing the marks made by m, m' the exact velocity of the shot can be calculated at all points of its course. The slide L is fixed parallel to F and the cylinder K by the brackets G, H. Y is a screw for drawing back the wheelwork M, and J a stop to regulate the distance between M and B. The depression of the lever / raises the two springs 8, which act as levers, and bring the diamond points m, m' down upon the paper. When an experiment is to be made, care is taken to see that the two currents are complete. The fly-wheel 4 is set in motion by hand, so as to make about three revolutions in two seconds. The markers m, m' are brought down upon the paper, and after four or five beats of the clock the signal to fire is given, so that in about ten seconds the experiment is completed, and the instrument is ready for another.

Fig. 2 gives a full-sized view of one of the markers, shewing the way in which it is moved. The depression of the lever h (Fig. 1), raises p, and thus the levers, which is formed of watch-spring wire, brings down m' to the paper, and keeps it gently in contact. This motion takes place within the circle k, about an axis CD. a' is an arm connected with the electromagnet. When the magnetism in E' is destroyed, a' begins to move away, and when it has moved a short distance it strikes the lever b′ a sudden blow which carries it as far as the hole in the stop c' will allow it to move. The The lever is rigidly connected with the circle k, which is capable of moving about an axis AB. This motion is communicated to m', which describes a very short arc of a circle about a point in AB. The arrangement is so made that when either of the markers m, m' is making a record, it has a motion which may be resolved partly in direction of the motion of the paper under it, and partly in a direction perpendicular to this. Thus records are obtained which can be read off by scale with great nicety.

Fig. 2.

S

The pendulum of a half-seconds clock strikes once each double-beat a very light spring, and so interrupts the galvanic current in E' once a second.