PHYSICAL ASTRONOMY. 157 
‘on the telescope, to balance the greater weight of the anterior half of the 
instrument; two, M and K, in the direction of the declination axis; and 
one, H, to diminish the friction of the hour axis. The clockwork, e, f, g, 
sles on the left side of the hour circle, d, and acts upon an endless screw 
which catches in it, and even without the clock-work, may be employed to 
produce a gentle motion about the hour circle. A second micrometer screw, 
i, is attached to the declination circle, k, to produce the adjustment in the 
declination. In a perpendicular position of the telescope, the height of the 
whole instrument amounts to 16 Paris feet, and the weight to about 4000 
Russian pounds. The steel hour axis, 39 inches in length, carries the hour 
circle of 13 inches diameter, with a graduation on silver on its lower face; 
each of the two verniers reads to within 4 seconds of time. The declination 
circle is 20 inches in diameter, and its motion is regulated by a clamp and 
micrometer screw. The circle itself has a graduation on silver, and the 
vernier reads to 10’. The length of the telescope is 1374 Paris feet, with 
a diameter of 10 inches at the upper end, and 7? at the lower. The object 
glass has a free aperture of 9 inches, and a focal length of 160. The four 
free oculars of 1420, 210, 320, and 480 magnifying power, as also the 
various micrometer apparatus with 14 oculars, can be screwed into the end 
of the draw tube. The aperture of the objective of 30 inch focus, belonging 
to the brass finder, amounts to 29 Paris lines. This finder has two oculars 
of 18 and 26 magnifying power. The clock-work intended to give to the 
telescope a motion on its hour axis, uniform with that of the fixed stars, and 
represented on a larger scale in jig. 20, consists of two principal parts; the 
clock proper and the wheelwork connecting this with the hour circle. 
Both are fastened to the bearing piece at the lower end of the hour axis. 
This wheel consists of an axis to which are attached the two wheels d and 
e; on d the disk f (the latter is omitted on the figure) is screwed ; a smaller 
disk, g, is attached anteriorly by the screw h. An endless thread with the 
weights runs over f and g. At 7 is seen the spring and trigger catching in 
the teeth of the disk g; dis in connexion with the little wheel 4; / and m 
are apertures. The clock drives an axle, n, with a double-motioned endless 
screw, working in the wheel, e. The motion of the clock is regulated by a 
centrifugal balance wheel which is connected with the weights by means 
of three wheels and pinions; p and q are cog-wheels, 7 a crown wheel 
acting on the pinion of the perpendicular axis t. The centrifugal balance 
wheel works within the box w; the parts, w, x, y, z, serve to regulate the 
motion of the clock, which, as well as that of the friction weight, continues 
to move more than an hour. Both the clock and the friction weight may 
be removed without disturbing the motion of the telescope. The micro- 
meter arrangements inside the telescope are adjusted with great accuracy, 
but their enumeration and explanation here would carry us too far out of 
our way. 
The value of this great refractor consists in its optical completeness, m 
the great accuracy of its adjustments, in the regularity of rotation about the 
hour axis by means of the clock, as well as in the perfectly unalterable 
‘micrometrical apparatus. In defining power and intensity of illumination, 
157 
