ASTRONOMICAL I \STRU.M KXTS. 3G9 



is moved up or down ; the centre part of the axis of this 

 arc is enlarged, and has an opening for receiving the 

 axis of the equatorial circles, to which they are firmly 

 secured. The equatorial circles are eight inches in di- 

 ameter, and consist of two brass plates with shampered 

 edges, the upper or vernier-plate turning freely on the 

 lower or graduated circle, having a horizontal motion by 

 means of a vertical axis ; this axis consists of two parts, 

 external and internal, the former secured to the graduated 

 hour-circle, and the latter to the vernier-plate. Their 

 form is conical, nicely fitted and ground into each other, 

 having an easy and steady motion when turned. The 

 equatorial hour-circle is divided into 720 parts, and num- 

 bered into degrees and half degrees, from to 360 ; also 

 into twice twelve hours, with the subdivisions reading to 

 two minutes ; these are subdivided by the vernier into 

 single minutes for the half degrees, and into four seconds 

 for the hour circle. In the upper edge of the vernier- 

 plate is cut, in its entire circumference, a thread, into 

 which works an endless screw, by which a slow and 

 steady motion may be given to the upper plate for any 

 number of revolutions. 



The semicircle of declination, 7J inches in diameter, is 

 attached to an axis, and by two standards supported and 

 fixed on the upper equatorial plate; it is divided into 

 two quadrants of 90 degrees each, subdivided to thirty 

 minutes, and having a nonius by which it may be read to 

 single minutes ; it has also a slow-motion endless screw, 

 by which it may be elevated or depressed ; on the upper 

 edge of this semicircle there is screwed a broad plate, 

 about twelve inches long, to Avhich the telescope is at- 

 tached, by means of two milled-head screws; the body 

 of the telescope is about four feet long, and the terrestrial 

 or long eye-piece about one foot long; the diameter of 

 the object-glass is three inches ; the power of the terres- 

 trial eye-piece is 50 times ; two celestial eye-pieces also 

 accompany the instrument, having powers of 120, and 180 

 to 200 times. Extra eye-pieces of intermediate powers 

 of 80 and 150 times may be had at an additional cost. 

 The adjustment of the eye-pieces to the focus of the 

 object-glass, is made first by sliding into a closely fitting 

 tube, as near as convenient for distinct observation ; and 

 then for fine adjustment, the eye-piece is moved in or out 



