Port of San Francisco

HARPER'S WEEKLY.

[DECEMBER 24, 1864.

828

FLEET OF WHALERS IN THE PORT OF SAN FRANCISCO.—[SKETCHED BY M. C. PALMER.]

FLEET OF WHALERS AT SAN
FRANCISCO.

WE give herewith an illustration representing the Fleet of Whalers in the port of San Francisco. Our correspondent, in a letter dated October 29, says that during a few previous days no less than twenty-four whalers had arrived from the Arctic ocean. The presence of so many whalers in the port at one time had never before been known ; perhaps half a dozen on one occasion has been the highest number previously reached. For several years past the whaling fleet has rather avoided San Francisco, and which is chiefly indebted for its recent presence to the facility afforded by this port for instant telegraphic communication with owners at New Bedford, whose wishes as to the disposition of cargoes can thus be immediately ascertained. The number of whales is said to be diminishing yearly, and eminent scientific authorities have advocated a cessation of the fishery for a period of years.

APPARATUS FOR MEASURING THE
VELOCITY OF PROJECTILES.

WE give on this page an illustration of the ELECTRO-BALLISTIC APPARATUS, recently invented by Major NAVEZ, of the Belgian Artillery, for measuring the space of time a cannon ball would take in passing over a few yards. Let two upright screens be placed before the gun, the first distant 30 yards and the second 150 yards. Between the screens is 120 yards : what time is taken by the cannon ball in passing from the first screen to the second? The same pendulum always performs its vibrations, whatever be the extent of the arc, in the same time. A seconds pendulum will tick seconds, whether the arc passed over be ten or forty degrees. If, then, a pendulum can be set in motion when the cannon ball penetrates the first screen, and stopped when the second screen is reached, the arc over which it passes during the interval would be an exact measure of the time

occupied. As the whole arc, fully completed, is to the partial arc, so is the time of describing the whole arc to the time of describing the partial arc.

The real thing to be accomplished is to start and stop the pendulum at precisely the right moment. This is accomplished by the principle of magnetism. Two pendulums are used, which, by means of a spring, are made to move in exact unison. One of these is called the main pendulum; the other, which is smaller and more delicate, the index pendulum. The main pendulum is connected with the first screen by means of a magnetic line, going and returning, so as to form a current. When the ball strikes the first screen the current between it and the main pendulum is broken ; this pendulum, which has previously been held by the magnet, is now, at the breaking of the current, set in motion, the index pendulum keeping exact pace with it. Now the pendulums are in motion; but when the ball strikes the second screen how are they to be stopped? A separate contrivance, called the Con

junctor, is connected with this screen by a magnetic current, which is broken by the passage of the ball. The Conjunctor is so contrived that, at the breaking of this current, a new current is established between a large magnetic battery and the index pendulum; and the moment this is accomplished the latter is stopped, being attracted to the iron arc along which it is moving. The new current was established by the pressure of a steel needle downward into a jar of mercury at the breaking of the current between the Conjunctor and the second screen. The problem is now solved. Suppose that, when the index is stopped, two degrees have been passed over of an arc which, completed, would measure forty degrees; and suppose that our pendulum ticks seconds. Then the time taken by the cannon ball in passing the 120 yards is one-twentieth of a second, one yard being passed in one-twenty-four-hundredth part of a second. By this contrivance a skillful operator is able to measure pretty accurately to the three-thousandth part of a second.

ELECTRO-BALLISTIC APPARATUS FOR MEASURING THE SPEED OF CANNON BALLS.