790

Meanings of Important Designations.

dicates the level of the horizon. During one night, thus, a person watching until dawn of the new day, may complete an interesting model of the whole sky around this earth, as existing both above and below the horizon, showing its most conspicuous stars in their true relative positions. In succeeding nights he may render the model more complete by adding other stars of inferior magnitude. Such a globe may be connected with a common clock, so as to turn round its axis once in twenty-four hours, and it will then exhibit uninterruptedly the changing places of all the stars marked upon it, whether they are above the horizon or below, and it will point through the day to the parts of the sky where the stars, although unseen then by the naked eye, are to be seen by means of a telescope properly directed.

1026. While the figure (fig. 309), half covered by a card up to the axis, exhibits the apparent motions of the sun, moon, and stars, all

vertical, as seen by persons near the equator, the adjoining figure half-covered indicates the apparent motions, all level, which are seen near the poles. The axle of the globe, when similarly half-coloured and placed obliquely, pictures the appearance at latitudes intermediate between the equator and poles.

Such a globe of wire with beads is a rough model of the so-called Celestial Globe, now part of the common furniture of our libraries. It has the advantage, by being transparent, of being able to show both sides of the globe at the same time, and to allow useful comparisons to be

The globe of wire, made like a bird cage, leads the spectator to appreciate at once the importance of being able to give a clear name or designation to every part of the surface of a globe, distinguishing it from every other part. When on a chess-board a square is said to be in some certain row from the top, and some certain row from either side, it cannot be mistaken for any other square. So on a globe; if a spindle pass through its centre, circles of wire may be fixed round it, at equal distances from the ends of the spindle (called poles, from the Greek, Toλéw, to turn), and they may be called circles of breadth or latitude; then other circles or lines of wire may be placed across these, cutting the equator at equal distances from one another, and reaching from pole to pole. These may be called

.

The Earth's Rotation illustrated.

791

circles of length or longitude. If all these circles have on them divisions into equal parts, all numbered, every spot or point on the surface of the globe will have its own latitude and longitude, perfectly distinguishing it from every other.

1027. A globe formed of white marble or other such material, if at rest, has no mark upon it to distinguish one part from another; but, if it be caused to rotate or whirl, every part instantly acquires certain peculiarities of motion, which clearly distinguish it. If a globe suspended by a twisted cord have small spots of any kind on its surface, as of ink spattered upon it, the spot to which the cord happens to be attached will appear to be at rest, as a pole of rotation, and every other spot will be describing a circle of latitude round the pole, which circle will be larger as it is further from the pole, until the equator, or the middle circle equally distant from both poles, is reached. A pen or pencil held to touch the turning globe at any spot would mark a circle of latitude there. If lines were then drawn directly from pole to pole across the equator at equal lateral distances, they would form lines of longitude.

It is readily perceived that the appearances of motion presented to the common eye among the heavenly bodies around the earth would be exactly the same, whether the bodies revolved round the earth as a centre, or the earth rotated as a central mass within a firmament at rest. It follows, therefore, that there are everywhere points or situations exactly corresponding in the starry concave above, and on the globular earth below. There must be in each hemisphere apparently fixed points, to be called poles, round which every other point will seem to revolve, and there must be perfect correspondence between what are called the latitudes and longitudes below and positions in the sky above. It is thus that a mariner bound to a small island like St. Helena, situated in the middle of a broad ocean, and which he cannot see until he arrives close to it, keeps his eye on the part of the sky which he knows to have the latitude of the island, and sails until he brings that part nearly over his head.

Among the general considerations bearing on the question of the earth's constant rotation on its axis may be mentioned the fact that all the other planetary bodies visible in the sky to our telescopes have such a rotatory motion, and even the sun himself is seen so to turn once in twenty-five days. Spots, which are frequently visible on bis surface, prove this.

792

Foucault's Experimental Proof

1028. A strikingly ingenious and beautiful experiment in proof was devised by M. Foucault, of the French Academy, as follows. If a pendulum is caused to swing, it performs its motion in one direction or plane, as from north to south, and will not deviate from that plane unless urged by some new force, even if its suspending thread is twisted. Accordingly, a pendulum hung from a support over the pole of a library globe set with its spindle vertical, will not be influenced by the turning of the globe beneath it, and will therefore show how far and how fast the globe may be made to turn under it. Similarly, a long pendulum caused to vibrate from a fixed support, over the centre of a large clock-dial laid on the ground near a pole of this earth, continues to move in the same plane, while the earth turns, and the line of the pendulum's motion continuing the same while the earth turns, indicates the fact, and the rate of the earth's rotation. If the experiment were made exactly at the pole, the apparent motion would be as rapid as that of the hand of a sidereal clock. At different distances from the pole it is proportionally less rapid.

A ball of metal suspended like a plummet from a point in a lofty ceiling will hang over a point in the floor which is directly beneath the point of suspension; but, if the ball be allowed to fall freely from the point of suspension, it will reach the floor considerably to the eastward of a mark on the pavement beneath the plummet. The reason is that the surface of the earth is rotating eastward, and as, in a turning wheel, parts distant from the centre have swifter motion than parts near to it, a body let fall from aloft, preserving its original onward velocity, will reach the floor in advance of where it would hang as a plummet.

1029. Many of the most remarkable motions observed on the surface of the earth, as the winds in the atmosphere and certain great currents in the seas, have their force and direction much influenced by the whirling of the earth. Explanations are given from pages 286 to 288.

The phenomena of the trade-winds, hurricanes, cyclones, &c., and of such movements in the ocean as the warm Gulf Stream, which sets across the Atlantic from the Mexican Gulf to the western shores of Europe, were little understood until within the last fifty years; and the value of the new knowledge may be judged of by the vast improvements in navigation made within that time.

Inspection of the globe sketched in Art. 1025, or of the common

The Astronomical Observatory.

793

celestial globe placed in libraries, explains readily the following terms-poles of the globe,-the equator,-latitudes and longitudes, the zenith, or summit of the sky, over the head of an observer, the nadir, a corresponding point directly below, the azimuth of a visible object, or the relation of its bearing to the cardinal points of the compass, north, south, &c.,—the horizon, or bounding level line on which the concave sky seems to rest.

The business of the astronomer is chiefly to study and measure the position and motions among the heavenly bodies, in relation to the poles, horizon, &c., as viewed from this earth, which is itself constantly in motion. By the apparent places of the heavenly bodies so observed, he learns the true places and motions, and can foretell what is to happen in future time.

A perfectly constructed globe of metallic circles, all accurately graduated or divided into the usual degrees, minutes, and seconds, with a good telescope at its centre, turning in any direction,—the whole being firmly supported on a suitable frame,—would enable an observer to make most of the measures which the astronomer desires to make, and might be called a small portable observatory. So delicate, however, and requiring such extreme precision, are many of the observations to be made, that it has been found necessary, instead of many small elements joined together, to have large instruments placed apart. A fit building erected in a suitable locality, furnished with such instruments, is called an Astronomical Observatory.

E

1030. The chief instruments in an observatory are, the large telescopes with graduated circles and the time-keepers. A good telescope can gather a thousand times more light from a distant object to form its image on the retina, than can enter the pupil of a naked eye. In the observatory are to be seen: Ist, the transit instrument, of which an outline is here given-a powerful telescope, A B, turning on a level axle, firmly supported at E and F, lying directly east and west, which causes the telescope to sweep along the meridian line from the south point to the zenith and beyond. Through this is watched the instant of a heavenly body's passing the meridian line. The field of view of the telescope is crossed by several vertical lines, and the instant being

Fig. 311.

H

794 The Earth's Rotation as a Mechanical Fact.

noted when the star passes each of these, the average of all gives the exact time. 2nd, the equatorial telescope, sketched in fig. 312. The telescope, A B, is supported on an axis which is made parallel to that of the earth, so that, as the earth turns, the telescope, having been pointed to any star or other object, follows that along its circuit. 3rd, the mural telescope, with large graduated circle. It has its axle fixed in the firm wall. It ascertains altitudes and polar distances very accurately.* 4th. the observatory clocks, showing both sidereal and solar time.

Σ

Fig. 312.

One matter of importance has as yet to be mentioned, which has to be allowed for by astronomers in their observations, namely, that the rays of light, which in empty space move straightly, do not so move when passing through the transparent medium of atmospheric air. They are then more or less bent or refracted according to the accidental temperature, moisture, density, &c., of the air at the time. A correction has to be made for refraction, as explained in the section on LIGHT, Art. 808, p. 578. In many astronomical observations, such as those made to determine the distance of the sun, a single second of an angular degree is an important quantity.

Mechanical View of the Earth's Rotation.

1031. The consideration of the original commencement of the earth's rotation belongs to the hypothetical speculations relating to the prior condition of the solar system. Once commenced, it goes on without abatement, so long as there is nothing to resist it. Although causes are supposed to be at work, tending to wear it away, as for example, the friction of the tides, yet there is no evidence of any sensible reduction in its rate within historical times. The effect of any loss of velocity would be to lengthen our day.

An important mechanical consequence of the rotation is the changing of the figure from an exact ball or perfect sphere to a flattened ball. If we measure the actual dimensions of the earth,

* In modern observations the use of the mural circle is abandoned, and a graduated circle is attached to the transit instrument, so that this instrument becomes capable of observing both time and altitude; it is then called a transit circle.