310 



NA TURE 



[July 26, 1894 



or slowness of combustion will make no difference in (he de- 

 veloped pressure, and that pressure will be the highest of which 

 the explosive is capable, regard being of course had to the 

 density of the charge. I say a small charge, because, if a 

 large charge were in question and explosion took place with 

 extreme rapidity, the nascent gases may give rise to such 

 whirlwinds of pressure, if I may use the term, that any means 

 we may have of registering the tension will show pressures very 

 much higher than would be registered were the gases, at the 

 same temperature, in a stale of quiescence. I have had in- 

 numerable proofs of this action, but it is evident that in a very 

 small charge the nascent gases will have much less energy 

 than in the case of a large charge occupying a considerable 

 space. 



The great increase in the magnitude of the charges fired from 

 modern guns has rendered the question of erosion one of great 

 importance. Few, who have not had actual experience, have 

 any idea how rapidly with very large charges the surface of the 

 bore is removed. Great attention has therefore been paid to 

 this point, both in regard to the erosive power of different ex- 

 plosives and in regard to the capacity of different materials 

 (chiefly different natures of steel) to resist the erosive action. 



The method I adopted for this purpose consisted in allowing 

 large charges to escape through a small vent. The amount of 

 the metal removed by the passage of the products of explosion, 

 which amount was determined by calibration, was taken as a 

 measure of the erosive power of the explosive. 



Experiments have also been maiie to determine the rate at 

 which the products of explosion part with their heat to the 

 surrounding envelope, the products of explosion being altogether 

 confined. I shall only briefly allude to ihese experiments, as, 

 although highly interesting, they have not been carried far 

 enough to entitle me to speak with coniidence as to final con- 

 clusions. 



Turning now to ballistic results. The energies which the 

 new explosives are capable of developing, and the high pres- 

 sures at which the resulting gases arc discharged from the 

 muzzle of the gun, render length of bore of increased import- 

 ance. With the object of ascertaining with more precision the 

 advantages to be gained by length, the firm to which I belong 

 has experimented with a 6-inch gun of loo calibres in length. 

 In the particular experiments to which I refer, the velocity and 

 energy generated has not only been measured at the muzzle, 

 but the velocity, and the pressure producing this velocity, have 

 been obtained for every point ol the bore, consequently the 

 loss of velocity and energy due to any particular shortening of 

 the bore can be at once deduced. 



These results have been obtained by measuring the velocities 

 every round at sixteen points in the bore and at the muzzle. 

 These data enable a velocity curve to be laid down, while from 

 this curve the corresponding pressure curve can be calculated. 

 The maximum chamber pressure obtained by these means is 

 corroborated by simultaneous observations taken with crusher 

 gauges, and the internal ballistics of various explosives have 

 thus been completely determined. 



Commencing with gun-cotton, with which a very Large 

 number o( analyses were made, with the view of determining 

 whether there was any material difference in the decomposition 

 dependent upon the pressure under which it was exploded, two 

 descriptions were employed : one in the form of hank or strand, 

 and the other in the lorm of compressed pellets. Both natures 

 were approximately of the same composition, of WaUham 

 Abf>ey manufacture, containing in a dried sample about 44 

 per cent, of soluble cotton and 956 per cent, of insoluble. As 

 ase'l, it contained about 2'25 per cent, of moisture. 



[Tables were given showing the results of the analyses of the 

 permanent gases.] 



From my very numerous experiments on erosion I have 

 arrived at the conclusion that the principal factors determining 

 its amount arc : (I) the actual temperature of the products of 

 combu.<ition ; (2) the motion of Ihese products. Hut little 

 T'. ive effect i* producetl, even by the most erosive powders, in 

 '•■ ■ vessels, or in ihoc portions of the chambers of guns where 

 •i.r motion of the gas is feeble or //iV ; but the case is widely 

 'lill'-tcnt where there is rapid motion of Ihe gases at high den- 

 sities. It is not difficult absolutely to retain without leakage 

 the products of explosions at very high pressures, but if there 

 be any appreciable escape before the gases are cooled they 

 inttanlly cut a way for themselves with astonishing rajjidily, 

 (olally destroying the surfaces over or through which they pass. 



NO. I 29 I, VOL. 50] 



Among all the explosives with which I have experimented 1 

 have found that where the heat developed is low the erosivj 

 effect is also low. 



With ordinary powders, the most erosive with which I am 

 acquainted is that which, on account of other properties, i; 

 used for the battering charges of heavy guns : I refer to browr 

 prismatic powder. The erosive effect of cordile, if considerec 

 in relation to the energy generated by the two explosives, i; 

 very slightly greater than that of brown prismatic, but very mucl 

 higher effects can, if it be so desired, be obtained with cordite 

 and, if the highest energy be demanded, Ihe erosion will bi 

 proportionally greater. There is, however, one curious anc 

 satisfactory peculiarity connected with erosion by cordite 

 Erosion produced by ordinary gunpowder has the most singula 

 effect on the metal of the gun, eating out large holes and form 

 ing long rough grooves, resembling a ploughed field in minia 

 ture, and these grooves have, moreover, the unpleasant habit 

 being very apt lo develop into cracks ; but with cordite, so fa 

 as my experience goes, the erosion is of a very differen 

 character. The eddy holes and long grooves are absent, and thi 

 erosion appears to consist in a simple washing away of th' 

 surface of the steel barrel. 



Cordite does not detonate ; at least, although I have mad 

 far more experiments on detonation with this explosive thai 

 with any other, I have never succeeded in detonating it. Will 

 an explosive like cordile, capable of developing enormous pres 

 sures, it is, of course, easy, if the cordite be finely comminuted 

 to develop very high tensions, but, as I have already explained 

 a high pressure does not necessarily imply detonation. 



[The velocities and energies developed by the new explosiv 

 were shown by the aid of diagrams.] 



"The Rotation of the Electric Arc." By Alexander Pelhai ^ 

 Trotter. 1 



In the course of experiments made with the view of realisin | 

 as a practical standard of light, the method of using one squaij 

 millimetre or other definite area of the crater of the positiv 

 carbon of an electric arc, ' the author has found that the effecliv 

 luminosity is not as theory would predict,- either constant ( 

 uniform. By the use of a doulile Kumford photometer, givin 

 alternating fields, as in a Vernon llarcourt photometer, h{ 

 attention was called to a bright spot at or near th| 

 middle of the crater. The use of rotating sectors accidental! ; 

 revealed that a periodic phenomenon accompanied the appea 

 ance of this bright spot, and although it is more marked wit 

 a short humming arc, the author believes that it is alwa; 

 present. 



An image of the crater was thrown on a screen by a photi 

 graphic lens ; and a disc having 60 arms and 60 openings of 3 

 and rotating at from 100 to 400 revolutions per minute, w^ 

 placed near the screen. Curious strobjscopic images we 

 observed, indicating a continually varying periodicity seldo 

 higher than 450 per second, most frequently about 100, difTict 

 to distinguish below 50 per second, and becoming with a loi 

 arc a mere flicker. The period seemed to correspond with til 

 musical hum of the arc, which generally breaks into a hiss at| 

 note a little beyond 450 per second. The hum is audible in 1 

 telephone in Ihe circuit, or in shunt to it. The current w 

 taken from the miins of the Kensington and Knightsbrldi] 

 Electric Light Company, often lale at night, after all tl' 

 dynamos had been shut down. The carbons were, of cottrs; 

 not cored ; six kinds were used. I 



A rotating disc was arranged near the lens, lo allow thebeaf 

 to pass for about i/ioooth of a second, and to be cut off I' 

 about l/ioolh of a second. It was then found that a brigj 

 patch, occupying about one quarter of the crater, appeared | 

 be rapidly revolving. Examination of the shape of this pat| 

 sh iwed that it consisted of the bright spot already nientione; 

 and of a curved appendage which swept round, sometini| 

 changing ihe direction of its rotation. This appendage seem: 

 to be approximately equivalent to a <|ua<lrant sheared ccl 

 ccntrically through 90°. Distinct variations in the luminos 

 of the crater are probably due to the (act that this is only 

 approximation, ' 



The d priori theory of the constant temperature of the oral' 

 is so attractive, that the .author is inclined lo altribute tl 

 phenomenon, not lo any actual change of the luminosity of I 



' J. .Swinburne and S. P. Thompsin, discussion on paper by llie .luil 

 '* Intt. Klecirical Kng.," vol. ai, pp. 384 and 403. 



' Ahncy and I'eslinK, I'liil. Trans. 1881, p. 890 ; S. P. Thompson 

 Artf. Journ, vol. 37, p. 333. 



