Page:The American Cyclopædia (1879) Volume XVI.djvu/306

 286 VELOCIMETER any other, is that invented by Capt. Le Bou- lenge of Belgium. A metallic standard, S, fig. 2, sustains two electro-magnets, A B. The magnet A, when excited, holds a bar C as its armature, called the " chronometer." A zinc tube, D, removable at pleasure, tits over the latter. The magnet B holds a smaller arma- ture, F, called a "registrar;" and immediately beneath it is an apparatus, L, holding in tension a cocked main spring, which carries a knife edge. After the rela- tive altitudes of the two magnets have been adjusted, the knife edge is caused to make an indenta- tion upon the zinc tube of the chronom- eter for a zero mark, and then the ma- chine is ready for the record. The magnet holding the chronom- eter is excited by the circuit through tbe screen nearest to the gun, and the registrar magnet by the other circuit. When the first is ruptured the chronometer falls ; when the second is ruptured the regis- trar falls, springing the knife edge and causing a cut to be made in tho zinc tube. The space be- tween this cut and the zero mark gives, by the formula for falling bodies, the time interval between the two ruptures. FIG. 2. The two machines described are capa- ble of showing the velocities of projectiles with great precision, their mean errors not exceeding -j-j.^y of a second. Col. Benton has used his machine without the aid of elec- tricity. In place of electro-magnets to hold the pendulums and release them, ho has em- ployed springs kept tense by means of cotton threads. The threads, being ruptured by the passing shot, release the springs, which dis- miss the pendulums. The results so obtained are but little less accurate than those with electro-magnets. Velocimeters are also used for obtaining data by which the resistance of air to the motion of projectiles may be determined. There is no known method of computing it, and the only resource is to measure it directly. This the velocimeter enables us to do by show- ing how much a projectile is retarded in pass- ing over a series of intervals. But for each set, and for each velocity, a separate trial must VELPEAU be made. The determination of the velocity through a succession of intervals requires a chronograph of much more complicated struc- ture than the simple ones described, for it must record the times of rupture of half a dozen cir- cuits, or of half a dozen ruptures of the same circuit. Usually the screens are all placed in the same circuit, which renews itself after each rupture, before the shot reaches the next screen. The recording device consists of a cylinder or disk revolving at a known rate, and receiving at the instants of rupture some visible marks from a stationary device or tracing point con- trolled by the current. The most ingenious application of the velocimeter yet made is the measurement of the varying velocity of a pro- jectile in the bore of a gun, which has been accomplished by means of a special apparatus devised by Capt. W. H. Noble of the British army, and also by a series of Le Boulenge chronographs. VELOCIPEDE (Lat. velox, swift, and pes, foot), a light carriage so constructed that it may be swiftly propelled by the feet of a person mounted upon it. In its earliest form it was invented at Mannheim in 1817 by Karl von Drais, and called a draitine. As then con- structed, it consisted of a bar about 5 ft. long and 6 in. wide, supported at each end upon a single wheel, the front one being so attached that it could be turned to the right or left like the front wheels of an ordinary carriage. The rider sat astride of the bar and propelled him- self and the machine by the action of his feet upon the ground. The vehicle in that form did not come into use, but about 1867 it was improved and became a favorite with amateur gymnasts. A saddle was fixed upon the longi- tudinal bar, and a foot crank was placed upon each side of tho forward wheel to serve for propulsion. The machine is kept in position by the action of the rider's body and limbs, and also by its own momentum in a certain plane. As recent examples of rapid riding tho .following may bo cited: 1 mile in 3 minutes, by J. Keen in a match with Frederick Cooper for the championship at Queen's grounds, Shef- field, England, Sept. 18, 1875 ; 50 miles in 3 h. 9 m. 21 a., by J. Keen at Molineux grounds, Wolverhampton, Nov. 80, 1874, including stop- pages; 100 miles in 7 h. 35 m. 48 s., by David Stanton, at Lillie Bridge grounds, Oct. 19, 1874; 132 miles in 17 h. 15 m., by J. T. John- eon, including all stoppages, from London to Worthington and back. VELPEAU, Alfred Armand Louis Marie, a French surgeon, born at Breche, department of Indre- et-Loire, May 18, 1795, died in Paris, Aug. 24, 1867. He was brought tip to assist his father, who was a farrier, learned almost without assistance reading, writing, and some of the rudiments of medicine, acquired reputation among the peasantry by several cures, and was enabled by a neighbor to study in the hospital of Tours, where he graduated in 1823. In 1830 he became surgeon to the Pitie hospital