February 17, 1898] 



NATURE 



;69 



cut at intervals along the path of the projectile, and as 

 each one is cut a knife is liberated which strikes the 

 shutter and imprints a mark upon it. These marks form 

 a record of the speed of the ball in terms of the known 

 speed of a falling body. 



This apparatus, which has been perfected by many 

 devices, is open to a fundamental objection. The release 

 of the shutter and of the knife takes time, a short time 

 may be, but an amount of time which becomes objection- 

 able when intervals as small as one-twentieth of a second 

 have to be dealt with. Projectiles nowadays attain 

 velocities of half a mile per second, and a new and more 

 accurate means is required for dealing with them. 



Such an instrument has been devised by Prof. Gushing 

 Crehore, of Dartmouth College, and Mr. Owen Squier, 

 lieutenant of artillery in the United States Army. It is 

 based upon an electro-optical principle of comparatively 

 recent discovery, that known as the Faraday effect. 

 When a beam of light is polarised by a Nicol prism and 

 then sent through another Nicol prism, it is transmitted 

 if the polarisation planes of the two prisms are parallel, 

 but is totally extinguished if the planes are crossed. If, 



and make a record of dots on the plate, which serve to 

 determine the time for the projectile record. The falling 

 shutter serves to limit the exposure to a portion of one 

 revolution of the plate, and to prevent the superposition 

 of records. 



The projectile, in starting from the mouth of the 

 cannon, breaks the first wire and the magnetising circuit, 

 thus extinguishing the light. When it reaches the second 

 wire, it restores the circuit in a manner explained by No. 

 2 (Fig. i). P. and B' are terminals of the circuit, A is an 

 insulating piece, and c c are elastic metallic plates kept 

 apart by the insulated wire D. The projectile in its 

 passage whips out the wire D, and allows c c to touch and 

 establish the circuit. The next wire breaks it again, the 

 next restores it, and so on. In the figure, four circuits 

 are indicated, which may be brought into play one after 

 the other, thus providing for the measurement of eight 

 time intervals. By placing these w;res at suitable distances 

 from the cannon's mouth, any portion of the path may be 

 minutely investigated. 



The most striking information derived from such 

 measurements is that the speed of the projectile goes on 



Fig. I. — Recording apparatus. 



T tube containing carbon bisulphide surrounded by magnetising coils, w camera, 

 closing circuit.] 



No 2, arrangement for 



however, a tube containing carbon bisulphide or other 

 rotatory substances is interposed between the crossed 

 Nicols, the light reappears as soon as the tube is exposed 

 to a strong magnetic field. This magnetic field is pro- 

 duced by a coil of wire surrounding the tube, and hence 

 it will be seen at once that an electrical contrivance is 

 possible which interrupts or restores the beam of light. 

 Add a photographic recording apparatus, and the scheme 

 underlying the Crehore-Squier arrangement is complete. 

 The recording apparatus is shown in Fig. i. p is the 

 polarising Nicol, A the analyser, T the tube containing 

 the carbon bisulphide and surrounded by the magnetising 

 coil, w is the camera containing the revolving sensitive 

 plate driven by the small motor m. The camera is pro- 

 vided with a falling shutter which covers a thin brass 

 disc provided with two small openings. One of these 

 openings admits the polarised light. The other admits 

 light which has had to pass through a hole in a piece of 

 aluminium foil carried on the prong of the vibrating 

 tuning-fork shown in the figure. The vibrations interrupt 

 and restore the light at perfectly constant known intervals, 



NO. 1477. VOL. 57] 



increasing for some time after it has left the cannon's 

 mouth. Starting with a muzzle velocity of about 480 m. 

 per second, the projectile increases its speed to, say, 

 515 m. per second in traversing the first 6 feet from the 

 mouth. It is only after having travelled some 25 yards 

 that the projectile is reduced to its original muzzle velocity. 

 This proves that the impulse of the expanding gas is felt 

 some distance along the path of the projectile. 



By a modification of the arrangement described, the 

 inventors have also succeeded in determining the speed 

 of the projectile inside the barrel of the gun. For this 

 purpose they secured a wooden lath to the cone of the 

 projectile (Fig. 2), provided with rings of metal at de- 

 creasing intervals. These rings were connected among- 

 themselves and to the projectile by a wire running along 

 the length of the rod, the whole being smoothed and 

 turned into an accurate cylindrical shape. A wooden 

 block, A (Fig 3), was fastened on the muzzle of the gun 

 by means of screw clamps, and a brass collar with steel 

 rings, c, was madeto.embrace the rod, which in the original 

 position of the projectile just projected from the muzzle. 



