of the large Refracting Telescope jii&t completed. 45 



wards the horizon, that its inclination may exactly correspond 

 with the latitude of the place, and is consequently directed to- 

 wards the pole. The second axis, called the axis of decli- 

 nation, is exactly vertical upon the first, or hour-axis. Thus 

 by directing the instrument upon a star, the hour-axis need 

 only be moved with that degree of velocity, as to make it turn 

 once within 24 hours, like the axis of the earth; by which 

 means tlie star will always remain in the field as long as it 

 stands above the horizon. This motion is imparted to that 

 axis by means of clock-work, consisting of two distinct parts. 

 The weight of the one part overcomes the friction and inertia 

 of the mass of several hundred weight ; the other part regu- 

 lates the motion. But in order to prevent a concussional mo- 

 tion, and make the same regularly uniform, the clock-work 

 was made without the usual pendulum, or balance. The re- 

 gulator of this work is a centrifugal pendulum, which, being 

 mclosed in a cone, constantly turns in one direction ; and both 

 the parts of the work may be wound up without the motion 

 of the telescope being interrupted in any degree whatever. 

 The telescope may also be stopped, and again set in motion^ 

 without any necessity of arresting the movement of the clock- 

 work ; and, if required, it may also be moved into any direc- 

 tion, either with the hand or by means of a screw. The mo- 

 tion of the clock may be at any moment accelerated or re- 

 tarded, \}y simply moving a spiral disk to a different degree 

 ot Its division. By this means a star may be moved to the 

 centre of the field of view, which is peculiarly useful in mi- 

 crometrical observations, and is not practicable in any other 

 manner. By means of this disk we may give the telescope 

 mstantaneously the movement corresponding with that of the 

 moon, or any of the planets. 



Ill order to render an uniform motion of the great telescope 

 possible, it must be completely balanced with respect to its 

 two principal axes, in whatever position it may be brought, 

 without however this balancing being an impediment to its 

 being directed towards any point of the sky that may be re- 

 quned. With respect to the axis of declination, the telescope, 

 not being fixed in the centre, is balanced by two weights 

 placed near the eye-glass and fastened to a conical brass tube, 

 each having in the point of gravity two axes intersecting each 

 other at right angles; so that in this respect the telescope is 

 equally balanced in every direction. With respect to the 

 hour-axis, the telescope is balanced by two weights, one of 

 which is fixed immediately on the axis of declination. The 

 second weight is fastened to a bar of a peculiar shape, form- 

 ing 



