ARTILLERY 251 



'driving ' bearing eight inches in advance of their seat. This latter point corresponds 

 nearly with that at which the front-studs hammer at starting. With shorter powder- 

 charges these several positions of the shot would be nearer the chamber. 



The longitudinal positions of the four cracks, four fissures, and the deep roughness 

 or erosion caused by the escaping gases, are shown by dark lines ; that of the greatest 

 enlargement of the bore by dark shading, at D ; those of the burrs on the edges of the 

 grooves are not stated in tne official report. The nature of these injuries would be 

 hardly visible on so small a scale, and the vertical positions could not be shown in a 

 section. Two of the cracks were on the lower side of the bore, all the other injuries 

 on the upper side, and their centres were 3 to 4 feet from A, where the greatest 

 powder-pressure occurred, but coincided with the point where the front-studs hammer 

 and the rear-studs come into ' driving ' bearing. The gun is being rebuilt, at a cost 

 of about 700J. or 800Z. 



In detail, the drawing shows the following effects : Four cracks, act, 1 5 J- inches 

 long, centre 44^ inches from the rear end of the bore at A ; bb, 3 inches long, centre 

 47 inches from A ; cc, 6J inches long, centre 45 inches from A ; dd, 6-J inches long, 

 centre 48 inches from A ; four fissures, c, 1| inches long, centre 40 inches from A ; 

 /, 1^ inches long, centre 42f inches from A; g, 2^ inches long, centre 44^ inches from 

 A ; h, 2 inches long, centre 48^ inches from A ; rr, a roughness extending from 1 inch 

 in front of the seat of the shot (28 inches from A), to 65 inches from A ; x, burrs, of 

 which the position is not specified, but approximately about 50 to 52 inches from A. 

 The powder-pressure is relieved at c, 33 inches from A (6 inches in front of seat of 

 shot) ; the greatest enlargement of the bore is at D, 42 inches from A (14 inches in 

 front of seat of shot) ; the hammering of front-stud is located 15 j inches away from 

 seat of shot, at 43^ inches from A ; and the rear-studs come into driving contact of 

 -an inch further on. 



In the ' Philosophical Magazine,' Captain Noble, of the Elswick Works, published 

 an elaborate paper on the influence of the spiral in rifled ordnance. The following 

 abstract of that paper, freed from its mathematical formulae, with the critical re- 

 marks, is from an experienced hand, 



'Every one is aware that elongated rifled projectiles are the subjects of two prin- 

 cipal motions during their exit from the bore, viz., a motion of translation and a 

 motion of rotation. If the rotation be efficiently accomplished, no other considerable 

 motion ought to take place. But if unmechanical devices be resorted to for the pur- 

 pose of effecting the revolution of the elongated projectile, other undesirable move- 

 ments are set up within the gun. However, under any sound mechanical arrange- 

 ment for supporting and rotating heavy elongated projectiles, it is an ascertained 

 'experimental fact, that the force necessary to impart rotation is only a small fraction 

 of that required to give a high velocity of translation. Hence it follows, both in 

 theory, and as an ascertained experimental fact, that the increment of gaseous 

 'pressure due to rifling is quite insignificant. It is with this small fraction of expul- 

 ' si vo force, and this insignificant increment of gaseous pressure, that Captain Noble 

 deals in his very neat mathematical investigation. He takes two utterly unmechanical 

 systems of rotating heavy projectiles one with an uniform angle of spiral, and the 

 other with an ever-changing angle both employing studs, and he yields the un- 

 mechanical precedence to the stud acting in an uniform spiral. Whether this be so 

 or not, is a philosophical question which appears to us to have been decided by Captain 

 Noble on erroneous date. Still, the formulas adduced will strike mathematicians as 

 particularly neat ; and, if they leave out of sight the whole of the practical objections 

 to the increasing spiral, those formulas cannot fail to form most instructive beacons to 

 all mathematical artillerists. 



'Captain Noble assumes that the action of studs within their grooves is uniform in 

 all the grooves. Now this cannot be, unless the major axis of the projectile coincides 

 with that of the gun throughout the whole of its transit. He also assumes that, 

 previous to starting, the studs are all in "driving" bearing, and at equal depths in 

 'their several grooves ; whereas the lower studs, on which the shot rests, and which 

 are the only points in contact with the bore, are then touching the "loading" side of 

 their groove, and all the other studs are more or less on the same, or " off" side of 

 their grooves. Before they can come into " driving " bearing, the shot must move 

 forward, slightly in the case of the uniform spiral, and about six or eight inches in 

 the parabolic groove. In the former cae, the so-called "pressure " is a succession of 

 'light "blows all round the bore ; and in the latter> the shot having attained about one- 

 third of its velocity, it is a succession of very heavy blows. Now. these successive 

 'blows are concentrated on one-iiich points of ench groove, and, constantly recurring 

 upon the same spot, produce slight enlargements and roughnesses, which prevent the 

 "smooth stud slipping away easily. The tendency of the shot's momentum, acting on 

 % circle of points near the centre of gravity and of-figuro, is to exert an effort of rota' 



