160 ASTRONOMY. 
Movable quadrants were also used ; and the constructions of Dollond ana 
Troughton (jig. 18) were the most convenient. The principal part con- 
sisted of the quarter circle, EI’, and two attached radii, IE and IF, perpen- 
dicular to each other, all of metal. Through the centre of gravity of the 
movable part of the whole instrument, passed a cylindrical tube, fastened to 
the quadrant IF KE, and including the axis of rotation; this gave off a 
vertical post resting on a solid base, AAD, adjustable in a horizontal plane 
by the screws B,B, B. This post was received in such a manner into a tube, 
C, fastened to the base, as always, in rotation, to preserve a vertical position. 
The body of the quadrant was united in such a manner to this part con- 
taining the axis of rotation, as to have its plane constantly vertical, and 
parallel to the axis of rotation. Upon the part containing the axis of 
rotation was attached the azimuth circle, DD’, graduated to ten 
minutes, readable to ten seconds by means of a vernier. The quadrant, 
divided to five minutes, was readable to single seconds by the micro- 
meter,G. The quadrant, with the movable telescope, KL, was so placed 
that one of the above mentioned metal radii was rendered perfectly 
horizontal by means of an attached level. H was a lens for reading off 
the graduation. Finally, as another means of determining the vertical 
position, a plumb-line was suspended in the above mentioned tube of the axis 
of rotation, whose proper position was given by four microscopes. K was 
the position occupied by the observer when looking at the stars through the 
telescope KL. 
The Transit Instrument, or Meridian Telescope. 
84. The transit instrument, one of the most important instruments of 
practical astronomy, was invented by Roemer in 1706. It is intended to 
obtain with greater accuracy the right ascension of a star, and consequently 
the solar time. It consists ( pl. 15, fig. 13) of an astronomical telescope, 
FD, fastened at right angles to a horizontal axis, B, and movable up and 
down in such a manner that the plane described always hes in the plane of 
the meridian of the place of observation. For the sake of the greatest 
possible firmness, the pillars, AA, upon which the two pivots of the horizontal 
axes rest, must be fixed separately, each one consisting of a single block of 
granite, and going deep into the earth, without any communication with the 
masonry of the building. 
Portable transit instruments have also been constructed, differing, how- 
ever, from the fixed only in their smaller size and their being adjustable to 
any point. Pl. 15, fig. 22, represents such a portable transit instrument. 
Although the transit instrument is not usually employed to obtain the meri- 
dian altitude of a star, yet for the approximate attainment of this end, 
a circle graduated to numbers is fastened to one side of the instrument in a 
vertical plane, as at I in fig. 13, and D, fig. 22. 
In the fixed, heavy transit instruments, it is absolutely necessary, for the 
purpose of lessening the friction, and the wearing of the pivots, to diminish 
160 
