that the pivot of the telescope is fixed, not to an immovable wall, but to a post which may turn upon its own vertical axis, and let this post stand on a horizontal circle having the same means for careful reading which have been described for circles in a vertical position. The tele

Fig. 40.

Railroad Transit.

scope may now be turned to any star; the vertical circle of the instrument shows the altitude of the star on a vertical circle of the sky, while the horizontal circle shows the bearing, or azimuth (12).

The common transit of the railroad engineer, when fitted with a vertical circle, is an altitude and azimuth instrument. The vertical circle takes elevations; the horizontal, bearings.

THE EQUATORIAL.

92. The telescope mounted equatorially.-When a star has been made to appear in the field of a telescope, it soon passes out of sight, because the earth moves the instrument past the star. The observer's attention is distracted by constant efforts to keep the star in view, and the difficulty increases with the magnifying power of the instrument. It is overcome by a system of machinery for moving the telescope, called an equatorial mounting. The principal pivot is placed parallel to the axis of the sky; it rests on the sloping face of a solid pier, usually

a single block of stone. This pivot is moved by clockwork, and turns the telescope westward as fast as the earth turns toward the east, thus counteracting the motion

tors are usually mounted equatorially, since they are used chiefly for studying the physical appearances of the heavenly bodies, and must command the entire sky.

93. As mural and meridian circles and transit instruments do not move out of a fixed plane, they require only

a narrow opening through which the stars may be seen. Equatorials are usually covered by a large hemispherical dome which has an opening at one side from the base to the top. The dome rests on rollers, and wheel-work turns it to present the window to any quarter of the heavens.

The instruments described are by no means all that may be found in large and well-appointed observatories. They include, however, the most important, and others differ in detail rather than in `principle.

A telescope contains a large lens, or a mirror, which furnishes an intensely bright image of a distant object, to be magnified by one or more lenses in the eye-piece.

The wires of the reticule determine the precise point observed. For measuring angles, the telescope is attached to a graduated circle, either vertical, or horizontal, or to both.

Observations of angles are made more accurate by the micrometer; of time, by connection with an electro-magnet.

The transit instrument observes the instant at which an object in the sky passes the meridian.

The mural circle gives the altitude of such a passage.

The altitude and azimuth instrument gives the place of a star at any time, and in any part of the heavens.

The equatorial mounting causes the telescope to follow a star for prolonged observation.

TIME.

CHAPTER VI.

LONGITUDE. RIGHT ASCENSION.

95. Definition.-Time is a measured portion of duration. It is measured by some kind of uniform motion. The ancients measured time by the flow of water from a vessel called a clepsydra, or of sand from an hour-glass. We measure time by the uniform beats of a pendulum,

or vibrations of a balance-wheel, as shown by the movement of hands over the dial-plate of a clock or watch. The standards of measure are found in the real or apparent motions of the heavenly bodies.

96. Natural units of time.-None of the most obvious events in the sky furnish an exact standard of time, because the portions of time marked by their recurrence are not of uniform length.

The natural day, whether reckoned from sunrise to sunset or from sunrise to sunrise again, varies in length. at different seasons of the year.

The changes of the moon, marking the period we call a month, do not occur at equal intervals, and it is difficult to fix by observation the exact instant of change. The division of the year into seasons is still more indefinite.

97. The solar day. For purposes of ordinary business, the passage of the sun over the meridian at noon is accepted as marking the middle of the day. The time from one passage of the sun over the meridian until the next, is called a solar day. As these intervals are not of uniform length, their average is a mean solar day. A clock which divides a mean solar day into twenty-four equal parts, called hours, is said to keep mean solar time.

98. Mean and apparent noon.-The instant when the sun crosses the meridian is apparent noon; the hour of twelve shown by a clock which keeps mean solar time is mean noon; it may be as much as 15 or 16 minutes earlier or later than apparent noon. The reason will be given in the articles on equation of time.

99. The civil day begins at midnight, 12 hours before mean noon, and ends at midnight, 12 hours after mean

noon.

100. The sidereal day. The successive transits of any fixed star, as observed by the transit instrument, occur at uniform intervals of 23 h. 56 m. 4.09 sec., mean solar time. This interval is the same at all seasons, and has not varied since astronomical observations began to be made. It furnishes the exact standard of time which we seek, and is called a sidereal, or star-day. It is the time occupied in one rotation of the earth.

THE ASTRONOMICAL CLOCK.

101. The sidereal or astronomical clock is so regulated as to divide a sidereal day into twenty-four hours. It keeps sidereal, or star-time. It is very carefully made that it may run with the utmost regularity, and it differs from a common clock only in keeping star-time, instead of mean solar time. With the telegraphic apparatus already described (82) it is of the highest importance in observing transits.

CELESTIAL CO-ORDINATES.

102. Apparent hourly motion of the stars. We have found (19, 20,) that the apparent motion of the stars is due to the actual rotation of the earth; that, while we speak of a star as coming to the meridian, as it seems to do, in fact the meridian sweeps by the star.

In 24 hours the earth completes one rotation. In that time, any place on the earth-the meridian of the observer has moved over 360°, passing all the celestial meridians (32) in succession. The meridian, therefore, moves eastward 360°÷ 24 = 15° in one hour, 15′ in one minute, 15" in one second.

103. Hence, if a star culminates (47) at eight o'clock, and another at 15 m. 45 sec. past 8, the second star is 15 m. 45 sec. of time, or 3° 56′ 15′′ of arc, east of the first.