520 Chronicles of Science. [July, 
A 20-inch gun too has been recently successfully cast at Pitts- 
burg on Rodman’s principle. In order to make this monster piece of 
ordnance, which will throw a solid shot of 1,000 pounds, 104 tons of 
metal were melted, though the gun will only weigh, when finished, 
56 tons. The essential feature of this system of casting is to cool the 
iron mass from the interior by means of a stream of water, which is. 
sent to the bottom of the bore in a properly protected pipe, while the 
exterior is kept hot by a fire round it. In this instance air was sub- 
stituted for the water after a certain length of time, as the water was 
found to lower the temperature of the metal too quickly. The run- 
ning of the iron occupied only 214 minutes, and the gun was ready 
for the lathe ina fortnight. These hollow-cast guns are also very 
durable, the 15-inch at Fortress Monroe having been already fired 
505 times. 
The use of gun-cotton, which is attracting so much notice on 
account of the Austrian experiments, has not met with favour. Up to 
the present, it has only been the solution in ether and alcohol that 
has been rendered available. By the aid of a coating of collodion, a 
cartridge of compressed gunpowder is made perfectly waterproof, and 
yet may be inflamed by a percussion cap without being torn open by 
the soldier. The advantage in rapidity of loading and freedom from 
dampness is very obvious. 
Much attention too has been directed to defensive as well as 
offensive warfare. Iron-plated vessels in large numbers have been 
built, the Government having a fleet of 75 on hand, or to be soon 
completed. The favourite style of protection has been with many 
layers of plates bolted or riveted together, and, where possible, 
backed with two or three fect of oak. A few vessels with solid plates 
of 44 inches thickness have been constructed ; but since it has been 
found that the 21-inch gun with cast-iron round shot would penetrate 
such armour, they are no longer regarded as perfectly protected. The 
‘ New Ironsides,’ a ship of this kind, has, however, done well, not less 
than ten 10-inch shot having struck her near the water line, without 
doing any serious damage. She has been hit 213 times without losing 
a man. 
When a person enters a Monitor turret, he cannot fail to feel a 
sensation of absolute protection, surrounded as he is on all sides by 11 
inches of iron. The only loss of life in these structures has been 
from boltheads flying off; but now that the use of through bolts has 
been dispensed with, this cause of insecurity no longer exists. In the 
iron-clad cruiser ‘ Dictator, —320 feet long, 50 feet beam, and 20 feet 
depth of hold—which is at present making ready for a trip to Europe, 
the turret has been increased to 15 inches, and the side plating to 
11 inches, with three feet of oak. She is expected to be quite fast, 
having two 100-inch cylinders of 4 feet stroke. The armament is 
only two guns, but they are of built-up wrought iron, and of 13 inches 
calibre. The maker, Mr. Ericsson, is to receive 1,000/. for every 
pound of powder over 50 pounds that they will burn. The risk of 
encountering a sea voyage has already been undertaken by Mr. Webb, 
who has just sent the ‘ Ré d'Italia, iron-clad 44, to Naples. She made 
