604 PHILOSOPHICAL TRANSACTIONS. [aNNO l/QS. 



rubbing plates, 3 feet 10 inches long, 1 feet 1 inch broad, and near 1 tenths 

 of an inch thick. These plates are fastened to the long bars of the tube, nearest 

 the top and bottom, by 6 arms each; and screwed on so as to be perpendicular 

 to the horizon. The plate on the west is fixed, but that on the east is adjust- 

 able, in order to be kept perfectly vertical on every part ot its surface. One of 

 these is visible against the tube, in the figure. An iron roller, 1 inch thick, 

 and 2-i- in diameter, is set in a strong frame, in such a manner as to allow the 

 claw, which holds it, to be set to any direction; where it can be afterwards 

 fastened by a large horned nut. This roller is mounted on a frame that may be 

 drawn up to any altitude, and lies on the whole set of ladders on the east; where 

 it rolls up and down on 6 sets of brass rollers. This machine consists of a 

 bottom frame, and a bar at rectangles to it, which, when the frame lies en its 

 rollers on the ladders, stands also at rectangles to them, on the lowest part of 

 the frame: it is braced so as to make the greatest resistance from east to west. 

 The bar carries the iron roller, which may be shifted to 2 different situations. 

 The latter is used in high altitudes. The iron roller, standing out, is then 

 turned so about as to be in the direction of the length of the eastern rubbing 

 iron; in which situation it is fixed by the horned nut. The telescope is then 

 brought forward or backward, by the bar machine, till the rubbing iron comes 

 to be opposite the roller. On one of the braces of this same frame is also 

 planted the nut belonging to the lateral screw motion, which has been described ; 

 and its long bar goes always with this machine, when it is disengaged from the 

 observing chair, and is laid back into a secure resting place. 



It will now be easily perceived, that when the eastern rubbing plate, in its 

 well adjusted vertical position, is pressed against the right ascension roller, by a 

 roller exactly opposite, and with a force sufficient to keep it firmly poised against 

 that roller, a vertical motion may be given to the telescope, in which the same 

 meridional situation will be preserved. Accordingly, I find that the right ascen- 

 sion of unknown objects, deduced from known ones, observed by the same 

 instrument, and in the same zone, is capable of great precision; and this con- 

 struction will therefore answer all the ends that were proposed. For it would 

 not be doing justice to the telescope to require of it all the accuracy of a transit 

 instrument. A machine, called a spring-bolt, is brought to any required situa- 

 tion by a rope fastened to the middle cross-beam of the stand, which comes 

 down, and goes through a pulley placed on the machine; in its return to the 

 top, it passes over a 2d pulley; and then goes down to a barrel with a wheel and 

 pinion, on the ground timber. The polar distance machine, as I call the oppo- 

 site one, on account of its chief use, which remains still to be explained, is drawn 

 up and down in a similar manner, by the handle of a pinion, wheel, and barrel. 



In the observatory is placed a valuable sidereal time-piece, made by Mr. Shel- 



