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POPULAR SCIENCE REVIEW. 
of catalogues, or for special purposes. Certain instrumental 
errors, such as flexure of the telescope, errors of circle divisions, 
and want of circularity of pivots, are determined occasionally. 
Nothing is supposed for a moment to he in perfect adjustment ; 
Fig. 3. 
THE ALTAZIMUTH. 
no'error is mechanically corrected ; everything is supposed to 
be always wrong, and to be corrected numerically in the after 
reduction of the observations. 
^But the Transit Circle has an important auxiliary in the 
Altitude and Azimuth, or, as it is briefly called, the Altazimuth 
instrument, which was designed specially for position observa- 
tions of the moon. Viewing the primary importance of these 
