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therefore the observer wishes to impart a rapid horizontal motion, he has only to apply to the telescope a force sufficient to cause the outside cone, together with the ring, to revolve upon the inside one; but when a slow motion is desired, it suffices that the endless screw be turned, in which case the ring, in con

Fig. 199.

TELESCOPE MOUNTED ON A PILLAR-AND-CLAW STAND, WITH FINDER, VERTICAL AND HORIZONTAL RACK MOTIONS, AND STEADYING RODS.

sequence of the friction, remains attached to the inside and immoveable cone.

Motion may be conveniently imparted in the horizontal direction by attaching to the endless screw a Hooke's joint mounted in a handle.

To all telescopes of a greater length than 3 feet, and mounted on a pillar-and-claw stand, steadying rods are a very desirable addition. They are generally 2 in number, and consist of 4 or more tubes sliding one within another, their ends terminating in universal joints fixed to the object-glass end of the telescope, and the 2 front "claw-legs" respectively. (Fig. 199.)

Fig. 200 represents a telescope and mounting, by Cooke and

Fig. 200.

TELESCOPE ON STAND, WITH VERTICAL AND HORIZONTAL RACK MOTIONS.

Sons, of York and London, extremely well adapted for general purposes, astronomical and terrestrial.

A contrivance known as Varley's Stand is sometimes used for telescopes longer than 4 feet; it is a very ungainly affair, and the regular equatorial stand is recommended for instruments too large to be conveniently placed on one of the pillar-and-claw construction indeed an equatorial may be said to be indispensable for the satisfactory conduct of observations in sidereal astronomy: without

it, much valuable time is apt to be lost in finding the objects sought after, which would be far more profitably spent in scrutinising a larger number found at once by the facilities afforded by graduated circles, &c.

Fig. 201 represents a mounting, having altitude and azimuth

Fig. 201.

TELESCOPE STAND WITH MOTION IN ALTITUDE AND AZIMUTH.

(Devised by Procter.)

motions, devised by Mr. R. A. Procter. The slow movement in altitude is given by turning the rod he; the endless screw on which, turns the small wheel at b, whose axle bears a pinion wheel working in the teeth of the quadrant a. The slow movement in azimuth is given in like manner by turning the rod he', the

lantern wheel at the end of which turns a crown wheel, on whose axle is a pinion working in the teeth of the circle c. The casings at e and e', in which the rods he and h'e' respectively work, are so fastened by elastic cords that an upward pressure on the handle h or a downward pressure on the handle h' at once releases the endless screw or the crown wheel respectively, so that the telescope is free to be swept at once through any desired angle either in altitude or azimuth. This method of mounting has other advantages the handles are conveniently situated and constant in position; also as they do not work directly on the telescope, they can be turned without setting the tube in vibration a.

a Pop. Sc. Rev., Oct. 1866, vol. v. p. 462.

CHAPTER III.

THE EQUATORIAL.

Brief epitome of the facts connected with the apparent rotation of the Celestial Sphere. -Principle of the Equatorial Instrument.-Two forms in general use.--Description of Sisson's form, and of the different accessories to the instrument generally.— Description of Fraunhofer's form of Equatorial.—In what its superiority consists. -The adjustments of the Equatorial.-Six in number.-Method of performing them.-Method of observing with the instrument, reading the Circles, &c.— Examples.

THE

HE reader is doubtless aware that the celestial sphere has an apparent motion of rotation round certain imaginary points in the heavens termed the Poles, one only of which is visible from any given point on the Earth, the equator excepted; that the altitude of the Pole, or angular elevation of it above the horizon, is equal to the latitude of the place of observation; and also that every star describes, apparently, a circular path around the Pole of the heavens, increasing in magnitude with the increase of the angular distance of the star from the Pole up to a distance of 90° or a quadrant, after which it again diminishes towards the opposite Pole.

If now we were to incline the pillar-and-claw stand already described, in such a manner that the vertical axis should point towards the Pole, and thus be parallel to the axis on which the sphere is supposed to revolve-in point of fact, if we give the pillar an inclination equal to the latitude of our station--it would clearly follow, that a single motion of the telescope, viz. one of rotation, about that inclined axis, would cause the line of sight (called also the optical axis) to trace upon the sphere a circle

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