July io, 1890] 



NATURE 



251 



made by the two small arms, which gave muzzle veloci- 

 ties of 1437 f.s. and I7i3f.s. When proper arrangements 

 were made, the photographs of the projectiles fired by 

 these two gims were always very fine and sharp. With 

 a sufficient velocity of the shot, the limit of the con- 

 densed air-wave in front of the projectile appeared to be 

 of a hyperboloidal form, whose vertex was in advance of 

 the projectile, and axis in the line of flight. Similar 

 traces in the photograph, indicating conical waves whose 

 axes were also in the line of flight, took their rise from 

 the base of the shot. Other but weaker traces of waves of 

 air took their rise from points on the surface of the shot. 

 All these straight lines in the photograph were inclined 

 to the line of flight at a rather less angle than the traces 

 of the head wave. When the velocity of the projectile 

 was increased, the angles which the traces of the waves 

 made with the line of flight were diminished. 



When the highest velocities were obtained, the chan- 

 nel vacated by the projectile was immediately filled with 

 peculiar little clouds, which appeared almost as regular 

 and symmetrical as beads strung on a line stretched in 

 the direction of the line of flight. And there was no 

 indication of a vacuum in the rear of the shot, even when 

 the velocity was so high as 900 m.s. (2953 f.s.). As the 

 air was transparent, the form of the waves of air in the 

 photographs must have been caused by the varying 

 density of the air, which refracted the rays of light. 



Long ago Robins noticed a change in the law of re- 

 sistance of the air to projectiles at about the velocity of 

 sound. Although Hutton disputed this change in the law 

 of resistance and others ignored it, recent experiments 

 have completely confirmed Robins's discovery. It now 

 appears that the disturbances caused by the projectile in 

 the air travel faster than the shot for low velocities, so 

 that the compression of the air in front of the projectile 

 is not sufficient to cause traces of waves in the photo- 

 graphs. 



The two guns which gave satisfactory results with 

 muzzle velocities of 1437 f.s. and 1713 fs., showed widely 

 different curvatures at the vertex of the wave of con- 

 densation in advance of the projectile. It was therefore 

 very desirable that the velocity of the shot should have 

 been exactly determined at the moment each photograph 

 was taken. This condition has unfortunately not been 

 sufficiently attended to, for although an improvised 

 ballistic pendulum was used in some cases, it was soon 

 discarded. 



Afterwards the experimenters made use of guns of 

 larger calibre. Salcher carried out experiments at Pola 

 with a gun of 9 cm. (3*5 inches) calibre, which gave a 

 muzzle velocity of 448 m.s. (1470 f s.). Other experiments 

 were made at Meppen, by Mach, assisted by his son, 

 with a gun of 4 cm. (r6 inch) calibre, which gave a 

 muzzle velocity of 670 m.s. (2198 f s.). The head wave 

 appeared as a stronger and broader hyperbolic curve in 

 the photograph, which was rather more in advance of the 

 head of the shot than in the case where small arms were 

 used. But when the velocities of the shot were nearly 

 the same in the two cases, the traces of the waves in the 

 photographs made nearly the same angle with the line of 

 flight. This perhaps might have been expected, as it has 

 been found experimentally that the resistance of the air 

 to projectiles varies as the square of their diameter. 



Further experiments were afterwards carried out in 

 the laboratory. In this case projectiles composed of 

 various metals were used, as brass, aluminium, and lead, 

 which were of various forms. Attempts were made to 

 determine the velocities of the projectiles in two different 

 ways, neither of which can be regarded as quite satis- 

 factory. In one case it was assumed that the work done 

 on the projectile by a given charge of powder would be 

 constant. But this assumption would not be true for 

 considerable variations in the weight of the projectile. 

 In the other case, the velocity was calculated by using 



NO. 1080, VOL. 42] 



the inclination, a, of the trace of the rear wave in the 

 photograph to the line of flight, on the supposition that 

 the velocity of sound = velocity of the projectile X sin a. 



Much labour and ingenuity have been expended in 

 bringing these experiments to their present satisfactory 

 state. The ground has been well prepared for sets of 

 systematic experiments made with useful forms of projec- 

 tiles fired with various muzzle velocities. The results 

 given by spherical projectiles might prove useful to the 

 theorist. Other experiments might be carried out with 

 ogival, hemispherical, and flat-headed elongated projec- 

 tiles. In all cases the readings of the barometer and 

 thermometer should be recorded, and the velocity of the 

 projectile should be measured. The ballistic pendulum 

 would probably give the best results if the block was 

 shielded from the action of the wave of condensed air 

 which accompanies the projectile. 



Further, E. Mach has attempted to compare the velo- 

 city of the report of a gun with that of the projectile. In 

 one series of his experiments, when the terminal velocity 

 of the projectile was higher than the normal velocity of 

 sound, the time of flight of the projectile, and the time in 

 which the report of the gun travelled over the same 

 distance, agreed very closely. But in another series, 

 where the terminal velocity of the projectile was below 

 that of sound, it was found that the time of flight of the 

 projectile was greater than that of the report of the gun 

 over the same distance. It was therefore considered that 

 the report of the gun travels at the same velocity as the 

 projectile so long as the velocity of the projectile is 

 greater than that of sound. But when the velocity of the 

 projectile is reduced by the resistance of the air below the 

 velocity of sound, then the report of the gun travels in 

 advance of the projectile, moving with the normal velocity 

 of sound. As experiments are frequently made with 

 velocities of the projectile more than double that of sound, 

 there seems to be no difficulty in the way of deciding 

 whether the report of a gun travels at the same velocity 

 as the projectile for high velocities. If so, as appears 

 probable, there arises the question as to the velocity with 

 which the report of the gun travels in various directions 

 from the muzzle of the gun. If a stretched membrane 

 could be made to interrupt a galvanic current for a 

 moment on the passage of a sound-wave, it would not be 

 difficult to determine the law of propagation of the report 

 of a gun in all horizontal directions. For the projectile 

 might be made to cut equidistant screens, and if lines of 

 properly prepared membranes, at the same distance apart 

 as the screens, were run in various directions, each fine 

 being provided with its own galvanic current and marker, 

 the progress of the projectile and of the report of the gun 

 in the chosen directions might be registered on the surface 

 of a cylinder rotating with a known velocity. B. 



NOTES. 



On Friday last, Mr. Isaac Roberts, F.R.S., of Maghuli, 

 Liverpool, was presented with an address on the occasion of his 

 removal from Liverpool to his new observatory near Tunbridge 

 Wells. The presentation took place in the Council Chamber at 

 the Town Hall before a large and representative assembly. The 

 Mayor (Mr. Thomas Hughes), who presided, referred in eulogistic 

 terms to the services rendered to astronomy by Mr. Roberts in 

 his chosen field of celestial photography. Principal Rendall 

 proposed the adoption of the address, in which reference was 

 made to Mr. Roberts's long and honourable business career in 

 Liverpool, and to the important discoveries made by him in 

 stellar photography. The address was signed by the Mayor, 

 Principal Rendall and the Professorial staff of University College, 

 many members of the City Council and of learned and scientific 

 Societies in Liverpool, and other prominent citizens. Mr. John 

 Hartnup, of the Bidston Observatory, seconded the motion, and 



