130 BENJ. PIKE'S, JR., DESCRIPTIVE CATALOGUE. 



bubble still remains central, the axis will be horizontal, but 

 if not, half the deviation must be corrected by the foot- 

 screw, b, and the other half by turning the small screw, i, 

 at one end of the level, which raises or lowers the glass- 

 tube (containing the air-bubble) with respect to its supports, 

 which rest upon the pivots. This, like most other adjust- 

 ments, frequently requires several repetitions before it is 

 accomplished, on account of the difficulty of estimating 

 exactly half the error. 



Having set the axis on which the telescope turns, parallel 

 to the horizon, and proved the correct position of the cen- 

 tral wire or line of collimation, making it describe a great 

 circle perpendicular to that axis, it remains finally to make 

 it move in that vertical circle which is the meridian. 



The correction of this error may be effected by turning 

 the screw, a, if the angular value of one revolution be 

 known, unless the instrument possesses an azimuth circle, 

 by which the telescope may be set exactly that quantity 

 from its present position. 



But if the quantity of motion to be given to the adjust- 

 ing-screw, a, is not a matter of certainty, the observer, after 

 ascertaining the difference of the intervals, must make the 

 adjustment which he considers sufficient, and again proceed 

 to verify it by observation, until, by continued approxima- 

 tion, he succeeds in fixing his instrument correctly in the 

 meridian. Price, according to size, <fec., $150 to $300. 



The Altitude and Azimuth Instrument. (Fig. 135, next 

 page.) To the centre of the tripod, A A, is fixed the verti- 

 cal axis of the instrument, of a length equal to about the 

 radius of the circle ; it is concealed from view by the ex- 

 terior cone, B. On the lower part of the axis, and in close 

 contact with the tripod, is centred the azimuth circle, C, 

 which admits of a horizontal circular motion of about three 

 degrees, for the purpose of bringing its zero exactly in the 

 meridian ; this is effected by a slow-moving screw, the 

 milled head of which is shown at D. This motion should, 

 however, be omitted in instruments destined for exact work, 

 as the bringing the zero into the meridian is not requisite, 

 either in astronomy or surveying ; it is in fact purchasing a 

 convenience too dearly, by introducing a source of error not 

 always trivial. Above the azimuth circle, and concentric 

 with it, is placed a strong circular plate, E, which carries 



