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DESCRIPTION OF THE OBSERVATORY AT CAMBRIDGE. 



The centre of motion of the whole instrument is twelve feet nine inches above the 

 floor of the dome. The entire length of the declination-axis from its junction with the 

 bed-plate of the tube is six feet ; the axis is of steel, or steel-cased, and its bearings are 

 three feet eight inches apart, and are relieved by four friction-wheels three and a half 

 inches in diameter. 



The declination-circle is twenty-six inches in diameter, reading by four verniers to 

 four seconds of arc. The polar axis is also of steel ; its length, from the end-bearing of 

 its lower pivot to its junction with the bed of the declination-axis, is three feet ten inches, 

 and its bearings are two feet eight inches apart. The upper bearing is relieved by two 

 friction-wheels seven inches in diameter, attached to the short arm of a lever, which moves 

 on a steel pin below the axis ; the longer arm of the lever passes through the upright part 

 of the cradle of the lower axis, and supports at its forked extremity the balance-weight 

 of the polar axis. The lower or southern end of the axis rests against a plate of hard 

 metal, which is capable of adjustment by means of a stout capstan-headed screw. 



The hour-circle is eighteen inches in diameter, and reads by two verniers to single 

 seconds of time. 



The cradle in which the polar axis rests is of brass, four feet ten inches long by nine 

 broad, and is four inches deep. This is secured to the bed-plate by twelve large steel 

 screws. The bed-plate itself is attached to the stone tripod by six screw-bolts. 



The tube of the telescope is of wood, veneered with mahogany and polished on 

 the outside. Within, it is lined with paper, and is strengthened by iron diaphragms. 

 The flexure of the tube is counteracted, and its balance preserved, by two brass rods 

 seventeen feet in length, having at their extremities nearest the eye-end brass spheres 

 filled with lead, eight inches in diameter. These rods turn on a universal joint near the 

 middle or centre of motion, and oppose the influence of gravitation on the longer and 

 heavier part of the tube in every position. 



The focal length of the finder telescope is forty-five inches, and its aperture three 

 inches. 



Sidereal motion is communicated to the telescope by means of clock-work, regulated 

 by the friction of centrifugal balls. There are eighteen eye-pieces of the following de- 

 scription. The powers given I have determined by means of a Dolland dynameter. 



Annular Micrometers. 



Plain Eye-pieces. 



No. 1. Power, 222. 



2. " 333. 



3. " 505. 



4. " 767. 



5. " HIS. 



