162 ASTRONOMY. 
tions of the two pillars, A, carrying friction rollers above, upon which the 
axis G rests. Fig. 34 is a handle for directing the telescope. 
Circular Instruments. 
85. As the irregularities. produced by changes of temperature, eccentricity, 
specific gravity, é&c., are greater in a part of a circle than in an entire one, 
it follows that even the most perfect quadrants do not afford the greatest 
possible degree of accuracy; for this reason full circles were introduced, 
now used almost exclusively in the determination of altitudes, and to which 
the remarkable precision of the astronomical observations of the present day 
is owing. To the circular instruments belong: 1, the repeating circle; 
2, the simple circle; 3, the meridian circle ; and 4, the theodolite. 
86. The repeating circle of Dollond (fig. 14), intended for observations 
out of the meridian, rests upon a tripod stand, AA, which, by means of the 
adjusting screws, and the level I’, can be made perfectly level. Upon this 
rests the horizontal circle, B, on which the direction of the alidade, H, can 
be accurately read off by means of four verniers provided with lenses: of 
these, only D and E are visible. The alidade H, which supports the four 
posts, I, I, 1, I, of the full circle, is adjusted by the telescope T.. The posts 
carry the beds for the horizontal axis of rotation of the two circles, O and L, 
and the principal telescope, M, whose horizontal position is regulated by the 
level attached to the strips K, K. The main telescope, M, has an adjustable 
ocular, provided with a micrometer arrangement. This telescope is fastened 
to the vernier carrier, PQ, which then determines altitudes in the fixed 
circle, O. The manner in which the repetition or multiplication is effected 
will be understood by referring to what has been said in the part of the work 
relating to measuring instruments (p. 65). We will Here only remark, | 
that for this purpose the vernier carrier, PQ, is fastened to the axis by 
clamps, which can be loosened in repeating. Reichenbach has very much 
improved the repeating circle; nevertheless, as there are always defects in 
the instrument, attention is now turned almost exclusively to simple fixed 
circles. 
The Meridian Circles. 
87. The most prominent and costly instrument of modern practica- 
astronomy, is incontestably the meridian circle, another kind of full circle, 
used to determine the altitudes of stars. This instrument serves not only to 
observe in the most accurate manner the culmination of the stars (as in the 
transit instrument), but also their zenith or polar distance. The entire 
instrument must therefore be set up in such a manner that its horizontal 
axis of rotation shall lie accurately in an east and west direction. The 
planes also of the two circles perpendicular to this axis, as well as the optical 
axis of the telescope, must be in the plane of the meridian. The meridian 
circle has much the same construction of the individual parts that is found 
162 
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