As we have said, the bore of a gun is rifled; that is, it is provided with spiral grooves that will set the shell spinning, so as to keep its nose pointing in the direction of its flight. Mortars, on the other hand, were originally intended for short-range firing, and their bore was not rifled. In recent years, however, mortars have been made longer and with rifled bores, so as to increase their range, and such long mortars are called "howitzers." The German 42-centimeter howitzer fired a shell that was 2,108 pounds in weight and was about 1-1/2 yards long. The diameter of the shell was 42 centimeters, which is about 16-1/2 inches. It carried an enormous amount of high explosive, which was designed to go off after the shell had penetrated its target. The marvel of this howitzer was not that it could fire so big a shell but that so large a piece of artillery could be transported over the highroads and be set for use in battle. But although the 42-centimeter gun was widely advertised, the real work of smashing the Belgian forts was done by the Austrian "Skoda" howitzers, which fired a shell of 30.5-centimeter (12-inch) caliber, and not by the 42-centimeter gun. The Skoda howitzer could be taken apart and transported by three motor-cars of 100 horse-power each. The cars traveled at a rate of about twelve miles per hour. It is claimed the gun could be put together in twenty-four minutes, and would fire at the rate of one shot per minute. FIELD-GUNS So far, we have talked only of the big guns, but in a modern battle the field-gun plays a very important part. This fires a shell that weighs between fourteen and eighteen pounds and is about three inches in diameter. The shell and the powder that fires it are contained in a cartridge that is just like the cartridge of a shoulder rifle. These field-pieces are built to be fired rapidly. The French 75-millimeter gun, which is considered one of the best, will fire at the rate of twenty shots per minute, and its effective range is considerably over three miles. The French supplied us with all 75-millimeter guns we needed in the war, while we concentrated our efforts on the manufacture of ammunition. GUNS THAT FIRE GUNS