Page:The American Cyclopædia (1879) Volume XVI.djvu/305

 VELLEIUS PATEKCULUS VELOCIMETER 285 9th centuries. The ruins of Velia are on a low ridge about 1 m. from the mouth of the Alento (anc. Hales), in the province of Princi- pato Citeriore and m. from the coast. A mediaeval castle and the village of Castella- mare della Bruce or Brucea mark the site of the ancient city. Excavations were under- taken here in 1874 by Salazaro. VELLEIIS PATERCULIJS. See PATERCULUS. VKLLETRI (anc. Velitrai), a town of central Italy, at the foot of Mt. Artemisio, in the province and 20 m. S. E. of the city of Kome ; pop. about 13,000. It has a fine cathedral and several public and private palaces. Wine and olives abound in the vicinity. Originally it was a Latin or Volscian city of considerable importance. According to inscriptions de- scribed by Cardinali (Rome, 1823), it had an amphitheatre and fine temples. Garibaldi de- feated the Neapolitans here in March, 1849. Velletri was the capital of a papal delegation till 1870, when it was incorporated with the kingdom of Italy. YELLORE, a town of British India, in the dis- trict of North Arcot, Madras, on the S. side of the river Palar, in lat. 12 55' N., Ion. 79 11' E., 79 m. W. S. W. of the city of Madras, and 16 m. W. of Arcot; pop. about 50,000. It is tolerably clean and well built, a place of con- siderable trade, and a station on the railway from Madras to the W. coast. About a mile N. of the town is an extensive fortress. In the public square there is a fine Hindoo pago.- da, built about four centuries ago. The climate is intensely hot, but healthful. Vellore was founded by the rajah of Bijanagur in the latter part of the 15th century. Sevajee took it from his descendants in 1677 ; and it fell into the hands of the British when they obtained pos- session of the Oarnatic. On the fall of Serin- gapatam it was selected as the residence of the sons and family of Tippoo Sultan. On July 10, 1806, the sepoys rose in mutiny at Vellore and killed 13 officers and 100 men of the Eu- ropean garrison. VELLUM. See PARCHMENT. VELOCIMETER, an instrument for measuring the velocity of projectiles. Prior to 1840 such measurements were made by suspending a gun in a pendulum, and observing the arc described in its recoil. This gave the means of compu- ting the velocity imparted to the projectile, with a probable error of a very few feet per second. In 1840 Wheatstone suggested the use of electricity for obtaining the data required for the computation, and for a quarter of a century it has been used exclusively. The ve- locity of a moving body becomes known when we know the time it takes to pass through a measured portion of its path. The space is determined by merely choosing certain points (usually about 100 ft. apart) in the path of the projectile, and measuring the distance be- tween them. The time required to pass over this distance therefore becomes the sole object of inquiry ; and a velocimeter ii merely an in- strument for measuring with extreme accuracy small intervals of time. To accomplish this, a screen of fine wire carrying an electric cir- cuit is placed at each end of the measured interval, in such manner that the passing pro- jectile shall rupture the two circuits, and the two ruptures instantly telegraph themselves to a machine, which records them in such a way that the interval of time between them can be immediately read. The recording instrument is called a chronograph. The number and variety of chronographs are very great, but they all involve one mechanical principle, viz. : the records must be made by means of parts of the machine moving at rates which are known with great exactitude, during intervals which begin and terminate with the two rup- tures respectively. This may be illustrated by the following contrivance, which is a modifica- tion of the Navez chronograph by Col. J. G. Benton of the United States ordnance depart- ment. It consists of a vertical metallic semi- circle, 5, graduated with the ordinary circular units and supported upon a bed plate, a. Two pendulums, p p 1, are swung upon the axis of the arc, and have their mass so distributed that FIG. 1. their times of oscillation are equal. "When de- flected in opposite directions from a vertical position to 90, they touch the magnets m m', which hold them in the new or horizontal posi- tion. The magnets are excited by circuits, c c, .c' c', which pass through the wire screens in front of the gun. If they were both ruptured at the same instant (the instrument being per- fectly adjusted), the pendulums would be simul- taneously released, and would pass each other opposite the zero mark at the lowest point of the graduated arc ; but being ruptured succes- sively, they pass at some point more or less distant from it, this distance being dependent upon the length of the interval of time between the two ruptures. To mark the point where this passage occurs, a stud, attached to the pendulum p, strikes the oblique head of a pin attached to a lever in the other pendulum, and causes it to make an indentation or an ink mark upon a piece of paper clamped to the graduated arc, leaving a record of the angle of deflection of the two pendulums at the in- stant of passage. By a simple formula the interval between ruptures can be computed from this angle. A very simple form of veloci- meter, probably used more extensively than