Now let us take a look at the breech-block of the 12-inch naval gun of to-day, which never blows out, not even when 350 lbs. of cordite go off just the other side of it. The explosion hurls an 850-pound shell at the rate of 3000 feet per second but it never stirs the breech-block. Yet it can be opened and closed so quickly, including the necessary fastening-up after closing, that shots can be fired from the gun at the rate of one every fifteen seconds. The breech-block partakes of the nature of a plug and also of a door. It swings upon hinges like the latter but its shape more resembles the former. If we want to make such a thing very secure we usually make it in the form of a screw with many threads, but that entails turning it round many times and that takes time. Given plenty of time to screw the breech-block into its place and there would never have been any anxiety as to the possibility of its blowing out, but there is not time. The problem, therefore, was to get the strength of a screw combined with quickness of action. This dilemma is avoided in the following simple manner. The breech-block is given a screw thread on its exterior surface, and the hole in the breech-piece is given a similar screw-thread on its inner surface, just as if the one were to be laboriously screwed into the other after the manner of an ordinary screw in machinery. Then four grooves are cut right across the threads on the block and similarly on the breech-piece, so that at four different places there is no thread left. In other words, instead of the thread running round and round continuously, each turn is divided up into four sections with sections of plain unthreaded metal in between. Thus in a certain position the block can be pushed into the hole without any threads engaging at all, for each strip of threaded block passes over an unthreaded strip in the hole and vice versa, in other words, the threads on the one part miss those on the other part. Yet an eighth of a turn serves to make all the threads engage and the thing is held almost as securely as if it were just an ordinary screw with threads its whole length. The block is carried upon a hinged arm so that although it can be turned in this manner it can also be swung back freely when necessary. Combined with the breech-block is a pneumatic contrivance which blows a powerful jet of air through the gun every time the breech is opened, thereby cleaning away the effects of the last explosion. Each of these great guns is mounted upon a slide so that when it is fired it can slide back, thereby exhausting the effect of the recoil, yet can be returned instantly to its original position. Indeed, this return is brought about quite automatically by the agency of springs, compressed air and hydraulic power. Thus the gun fires, slides back, returns and is at once ready for the next shot.