330 



FRENCH EXHIBITION. 



per knot. The Atlantic shore end is also very 

 strong, and there is another powerful cable 

 made of four recovered cables, originally laid 

 to Hague by the Electric Telegraph Company, 

 laid around a core of Hooper's wire, and now 

 submerged between Scotland and Ireland. 



Siemens has also a neatly-arranged case of 

 cables, exhibiting the various patterns con- 

 structed by him. Their form of deep-sea cables, 

 though not strictly novel, is so far peculiar and 

 original that it deserves notice. The core is 

 covered with a double strand, in opposite direc- 

 tions, of hernp ; and this hemp serving, covered 

 with a sheathing of copper ribbons, spirally 

 laid and overlapping each other. 



Insulating Material. Nothing is more as- 

 tonishing than the progress made in this branch 

 of telegraphy. The great perfection to which 

 the science of testing has been brought has 

 probably conduced more to this than any thing. 

 Samples of insulated wire that in 1862 would 

 have been considered perfect would now be 

 considered worthless. 



Eattier and Co.'s samples of gutta-percha- 

 covered wire quite equal in appearance any 

 thing produced in England. Eattier tests all 

 his wires in water, but not under pressure, by 

 means of Wheatstone's balance. Wire of No. 6 

 ordinary gauge outside measurement gave, ac- 

 cording to their own printed statement, about 

 300 million ohms per mile. This is very good 

 indeed, but not equal to the productions of 

 Wharf-road and Silvertown, where they now 

 obtain 3,000 million ohms, and even more, for 

 the same wire. Hooper exceeds even this. Its 

 peculiarity is this : it has a layer around the 

 conductor of the best and purest Para rubber, 

 and its exterior coating is a coarser kind, and 

 vulcanized ; but these two layers are separated 

 by a third layer, composed of a mixture of rub- 

 ber and oxide of zinc, which is said to perfectly 

 protect the inner layer from the injurious effect 

 of oxygen. 



Electro - Ballistic Apparatus. The "gun 

 pendulum," proposed by Eobins, but first used 

 by Count Eumford about 1780, had measured 

 the velocity of the shot by the recoil of the 

 piece from which it was fired, the velocity 

 itself could not be measured, but only the 

 eifect of the striking force, which was then 

 resolved into weight, form, etc., of projectile, 

 and then a calculation was made to ascertain 

 what must have been the velocity to produce a 

 certain effect. Professor Wheatstone proposed, 

 in 1840, electricity to measure the velocity of a 

 shot, but the first instrument was constructed 

 and practically used in 1849, by Captain Navez. 

 Its principle is roughly this: An electro-mag- 

 net is arranged in such a position that it holds 

 up a pendulum so long as the galvanic current 

 is passing through the'magnet. When the cur- 

 rent is broken the pendulum falls, earring with 

 it a loosely attached index-needle, which passes 

 rapidly over a graduated arc. The moment a 

 current is reestablished the needle is checked 

 in its career, while the pendulum swings free of 



it. Having previously ascertained exactly the 

 time required by the pendulum to pass over 

 such an arc, the time between the breaking 

 and re-making of the current is known. It 

 only remains, therefore, to erect two targets, 

 mere frameworks of wood, with the circuit 

 wire led backward and forward across them, 

 and to arrange so that the shot, passing through 

 the first "screen," as it is called, shall interrupt, 

 the galvanic current, and afterward reestab- 

 lish it by passing through the second screen. 

 Vignotti simplified the apparatus and marked 

 the points of rupture of the two targets by 

 means of sparks which passed with the rapidity 

 of thought from the pendulum through a piece 

 of prepared paper, leaving thus two traces of 

 miniature lightning flashes corresponding to the 

 passage of the shot through the two screens. 

 The breaking of a wire at the muzzle of the gun 

 as the shot came out released the pendulum, and 

 the sparks were delivered when it was in full 

 swing. Major Benton, an American officer, 

 devised an instrument in 1859 which carried 

 two pendulums, one for each screen, and re- 

 quired two galvanic batteries to work it. The 

 batteries were brought to the same strength, 

 being tested by allowing the pendulums to 

 drop from opposite ends of the arc, when they 

 should meet exactly in the centre. Wherever 

 they meet a mark is made by mechanical means. 

 On firing the gun the pendulums drop at the 

 first and second screen, are pierced respectively, 

 and the difference between the spaces passed 

 over by the two before meeting gives the arc 

 due to the space been the targets. Colonel 

 Leurs, of the Belgian Artillery, has introduced 

 an improvement on the Navez apparatus, usu- 

 ally known as the Navez-Leurs machine. It 

 is on the same principle as Benton's, differing 

 from the latter chiefly in its mode of register- 

 ing the time of meeting of the pendulums. In 

 Bashforth's and Schultz's instruments the flight 

 of time and that of the shot are registered side 

 by side upon a revolving cylinder. In the 

 " Schultz chronograph " a tuning-fork vibrates 

 under the influence of two electro-magnets, 

 one outside each branch of the fork. The left 

 branch carries a fine quill-point, which traces 

 a line upon the surface of the revolving cylin- 

 der, previously blackened by holding it over 

 the flame of a lamp. The cylinder receives 

 motions of rotation and translation by means 

 of a system of clockwork. When the fork is 

 at rest and the cylinder rotates, a line will be 

 drawn round the latter in the form of the helix 

 of a screw. This having been done, the motion 

 is stopped and the cylinder replaced in its first 

 position. Again the clockwork is set. in mo- 

 tion, and this time the fork is made to vibrate, 

 tracing with its quill-point a sinuous line which 

 crosses the first drawn helix once for every 

 vibration of the fork. The number of vibra- 

 tions to one second being known, the wavy 

 line becomes an extremely accurate and minute 

 scale of time. A number of screens are so ar- 

 ranged that the galvanic current passes through 



