EOCKBEIDGE EOCKET 367 swine ; 8 manufactories of agricultural imple- ments, 23 of carriages and wagons, 6 of cheese, 5 of furniture, 4 of iron castings, 3 of machi- nery, 8 of brick and stone, 3 of sash, doors, and blinds, 3 of woollen goods, 8 flour mills, and 4 distilleries. Capital, Janesville. II. The -S. W. county of Minnesota, bordering S. on Iowa and W. on Dakota ; area, 432 sq. m. ; pop. in 1870, 138. It has a rolling surface, and is drained by tributaries of the Big Sioux and Eock rivers. ROCKBRIDGE, a central county of Virginia, intersected by North river, a branch of the James, and bordered S. E. by the Blue Eidge ; area, about 700 sq. m. ; pop. in 1870, 16,058, of whom 3,890 were colored. It has a moun- tainous surface and very fertile soil. The Chesapeake and Ohio railroad crosses the N. W. corner. The county derives its name from the natural bridge in the S. corner. (See BRIDGE, NATURAL.) The chief productions in 1870 were 214,800 bushels of wheat, 119,518 of Indian corn, 85,564 of oats, 6,022 tons of hay, 186,469 Ibs. of tobacco, 9,156 of wool, 131,092 of butter, and 3,290 gallons of sor- ghum molasses. There were 2,288 horses, 2,378 milch cows, 4,493 other cattle, 3,481 sheep, and 6,986 swine ; 1 manufactory of ce- ment, 2 of tin, copper, and sheet-iron ware, 4 of tanned and 4 of curried leather, and 16 flour mills. Capital, Lexington. ROCK. CASTLE, a S. E. county of Kentucky, bordered S. E. by Eock Castle river, by the branches of which and Dick's river it is drain- ed; area, about 350 sq. m. ; pop. in 1870, 7,145, of whom 369 were colored. It has an uneven surface and a not very fertile soil. It is intersected by the Louisville and Nashville and Great Southern railroad. The chief pro- ductions in 1870 were 10,539 bushels of wheat, 216,816 of Indian corn, 35,077 of oats, 23,445 Ibs. of tobacco, 10,949 of wool, 92,675 of but- ter, and 11,702 gallons of sorghum molasses. There were 1,638 horses, 1,648 milch cows, 2,314 other cattle, 6,625 sheep, and 7,189 swine. Capital, Mount Vernon. ROCKDALE, a N. county of Georgia, bound- ed S. W. by South river, a branch of the Oc- mulgee, and drained by other branches of that stream ; area, 200 sq. m. It has been formed since the census of 1870. The Georgia rail- road traverses it. The surface is diversified, and the soil good. Iron, gold, and other min- erals are found. Capital, Conyers. ROCKET, a projectile which is set in motion by a force residing within itself, thus perform- ing the twofold functions of piece and pro- jectile. Eockets are used as night signals, mis- siles of war, and in pyrotechnic displays. They are said to have been invented about the close of the 9th century, and to have been used for war purposes in India and China even before the invention of gunpowder. Their inferior f orc and accuracy limited their use by Euro- peans to incendiary and signal purposes till 1804, when Sir William Congreve turned his 708 VOL. xiv. 24 attention to their improvement. He substitu- ted sheet iron for paper cases, made the guide stick shorter, and attached it to the centre instead of the side of the base. He prepared and used them successfully at the siege of Bou- logne and the battle of Leipsic, and is said to have increased the range of the six-pounder rocket from 600 to 2,000 yards. Eockets are now constructed of cylindrical cases composed of paper or wrought iron, according to the use for which they are intended, and are filled with a composition of nitre, charcoal, and sulphur, like gunpowder, except that the ingredients are compounded for a slower rate of combus- tion. If penetration and range are required, as in the case of war rockets, the head is sur- mounted by a solid shot; if explosion or in- cendiary effect, by a shell or a case shot with a fuse attached, which is ignited by the flame of the composition. The base is perforated by one or more vents for the escape of the gas generated within, and sometimes with a screw hole into which a guide stick is fastened. A rocket is set in motion by the reaction of a rapid stream of gas escaping through the vents and impinging against particles of the air, al- though the reaction of the escaping gas would cause it to move also in vacua. The velocity of the flight depends upon the size and shape of the vents and the velocity with which the gas escapes, and it has been found in practice that the best results are obtained by conical vents. As the composition burns in parallel layers of uniform thickness, the amount of gas generated in a given time or the velocity of its escape from the case depends on the extent of the inflamed surface; and experience shows that to obtain the required surface of inflam- mation it is necessary to form a long cavity in the mass of the composition. This cavity is called the bore, and in small rockets it is obtained by driving the composition around a spindle, which is afterward withdrawn, while in large rockets -the composition is driven in solid, and afterward bored out; the bore is made concentric with the case, and slightly conical. The propelling force of a rocket changes its direction with the axis along which it acts, so that without means of giving sta- bility to this axis the trajectory will be very irregular. Instances have been known where these projectiles have returned to the point whence they started; the "serpent," a species of small rocket, owes its peculiarity to this fact. The two means used to give steadiness to the flight of rockets are rotation, as in the case of a rifle ball, and the resistance of the air, as with an arrow. The first is exempli- fied in Bale's rockets, where rotation is pro- duced by the escape of the gas through vents situated obliquely with reference to the axis. The common signal rocket is guided by a long stick projecting from its base in the continua- tion of its axis, or by four shorter ones at- tached to the side of the case at its base, and each making a small angle with the axis.
