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"PENCIL

1444

PENGUIN

ing indented by several bays. The surface is intersected by a range, the highest point of which is about half a mile above the sea. The soil is fertile, and rice, tapioca, pepper, cloves, nutmegs and other tropical fruits and vegetables are grown. Georgetown, in the north, is the capital. It has a good harbor and considerable trade, Adjoining native states are under British protection, and contain the richest tin-fields in the world, besides valuable forests yielding rubber, gutta-percha and gums. In 1906 the exports were $90,709,225, and the imports $94,546,112.

Pen'cil. A slender stick of black lead, slate or chalk, inclosed in a round piece of wood is called a pencil, but the term is also applied to small hair-brushes, used by artists, and to these the name was originally given. For a long time graphite or plumbago from the Cumberland mines in England furnished the "leads" for the best pencils ever made; and since these mines were exhausted vast quantities of the same material have been found in Siberia. By a method patented by Brockedon in 1843 ^m"s material is freed from impurities by grinding it to powder, and is then formed into solid blocks by subjecting it to heavy pressure in cases from which the air is removed. The manufacture of black lead and colored pencils is carried on extensively at Nuremberg, where there are more than twenty factories, which employ several thousand hands and annually produce about 250,-000,000 pencils.

Pen'dulum, literally a hanging body, is used in physics to denote any body performing isochronous or nearly isochronous vibrations. (Isochronous means equal-timed). Thus a magnet freely suspended so as to vibrate in a horizontal plane is sometimes called a magnetic pendulum. An ordinary clock pendulum is called a gravitational pendulum. A brass rod, so suspended by a wire as to vibrate in a horizontal plane, is generally known as a torsion pendulum. Space permits us to discuss only the gravitational pendulum. This simple but elegant instrument serves two principal purposes, each of which was first pointed out by Huygens, the great Dutch physicist (3[673;. One use is that of a time-measurer, an application based on the fact that, so long as a pendulum remains of constant length and swings through the same angle, it vibrate® at a constant rate. In practice its length is kept constant by "compensation" and its angle of swing is kept constant by means of a spring which gives it a little push at each vibration. (See CLOCK.) The second use is as an instrument for measuring the acceleration of gravity at various points over the surface of the earth. This is generally done in two ways* (i) By suspending a heavy metallic sphere of known radius by means of a fine wire of known length and observing the period of vibra-

tion; or (2) by suspending a bar of * metal from one of two such points that it has the same period of vibration from whichever point it be suspended. The former method is a near approximation of what is called a simple pendulum, namely, a heavy particle suspended by a massless thread; the latter is a reversible compound pendulum. It can be proved by dynamics that the period, T, of a simple pendulum whose length is I, is given by the equation

where g is the acceleration of gravity at the place of observation. Using this equation for the brass ball, suspended by a wire, and making some slight corrections for the wire and for the diameter of the ball, one may obtain quite an accurate value of the acceleration of gravity. This method is due to Borda. In the case of the reversible pendulum it can be shown that the distance between the two points of suspension is exactly equal to the length of a simple pendulum which would vibrate with the same period. Hence, to obtain g with great accuracy, one has only to measure this distance, which we may call I, determine the period, T, and solve for g the equation given above.

The pendulum is frequently employed also to compare the acceleration of gravity at several different places. Here we may disregard the length of the pendulum, provided this remains constant, since the ratio of the acceleration at two stations depends only upon the square of the ratio of the periods at the same two stations. For this purpose the United States Coast Survey uses pendulums which are very short and convenient, beating quarter-seconds.

Penguin (pen'gwtn), a swimming bird peculiar to the southern hemisphere. The birds are diving swimmers. They are very awkward on land, but wonderfully expert in water. It is said they can outswim fish. Their wings, which do not suffice for flying, are paddle-shaped, and in swimming are brought alternately into use. In diving and in swimming under water only the wings are used, the feet serving as a rudder. The feet are placed so far back on the body that the bird is erect when standing. They have a smooth, scale-like plumage adapted for slipping through the water. The birds live mostly on the water and go on shore

KING  PENGUIN