MECHANICS AND USEFUL ARTS. 117 



NEW LIGHTHOUSE APPARATUS. 



Iii the Lochindaal Lighthouse, in the island of Islay, Argyle- 

 shire, dioptric prisms of a new form have been introduced. The 

 light which passes behind the flame has hitherto been sent forward 

 by two optical agents, so as to mingle with the front light, and thus 

 to reach the eye of the mariner; but the object is now effected for 

 part of the upp'er core of rays by means of the new prisms alone, 

 so that one agent is saved, and the loss of light by absorption and 

 superficial reflection is prevented. 



The prisms act by refraction and total reflection, and consist of 

 glass of the ordinary index of refraction. By means of the prisms 

 .and a spherical mirror, the whole of the back light is sent for- 

 ward. 



THE WORKING OF BRONZE. 



The secret of the manufacture of Chinese gongs seems to have 

 been revealed recently by MM. Julien and Champion, who have 

 found that bronze, which is brittle at the ordinary temperature, 

 becomes malleable at a dull red heat. Experiments lately made 

 on this matter at the Paris mint, with the view of determining the 



^j 



conditions most favorable to working the alloy, found that a 

 bronze containing 20 per cent, of tin, which at the common tem- 

 perature is as brittle as glass, may, at a dull red heat, be forged 

 and beaten out as easily as soft tin. 



IRON SHIPS. 



Iron for ships is rapidly superseding wood in English ship- 

 yards. In 1865 there were 806 wooden ships built in England, 

 in 1869 but 324. In 1868 the tonnage of iron ships built was 

 235,937, against 66,977 wooden, and 24,121 of composite. Iron 

 ships are more durable, require less repairs, and stand heavier 

 storms than those of wood, and it will not be long till the latter 

 must be entirely superseded. 



GUNNERY EXPERIMENTS. 



The London " Globe " details some late artillery experiments 

 which showed that, in spite of all possible care in the arrange- 

 ment, the exact level of the centre being taken on the target by 

 means of a theodolite, the shot would strike 10 inches above it. 

 Theoretically, the shot would fall by gravity, and its centre 

 should have struck about 2 inches below the level. The probable 

 explanation is, that the recoil is sensibly felt before the shot has 

 left the gun, and that the resultant of the forces acting on the gun 

 and carriage tends to throw the muzzle up ; thus the projectile, 

 although seemingly fired point blank, really leaves the gun at an 

 angle. With the 12-pounder breech-loading gun this angle was 

 found to equal about 30 minutes, while with the 9-pounder muz- 



