3po Dr. Herschei/s Description of a 
which the same meridional situation will be preserved. Ac- 
cordingly, I find that the right ascension of unknown objects, 
deduced from known ones, observed by the same instrument, 
and in the same zone, is capable of great precision ; and this 
construction will therefore answer all the ends that were pro- 
posed. For it would not be doing justice to the telescope to 
require of it all the accuracy of a transit instrument. 
The spring-bolt, as I call this latter machine, is brought to 
any required situation by a rope fastened to the middle cross- 
beam of the stand, which comes down, and goes through a 
pulley placed upon the machine ; in its return to the top, it 
passes over a second pulley, and then goes down to a barrel 
with a wheel and pinion, on the ground timber at Q'. 
The polar distance machine, as I call the opposite 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 placed at R'. 
In the observatory is placed a valuable sidereal time-piece, 
made by Mr. Shelton, for which I am obliged to my astrono- 
mical friend Mr. Aubert, as a gift that will always be highly 
esteemed. Close to it, and of the same height, is a polar 
distance piece, which has a dial-plate of the same dimensions 
with the time-piece ; and is also divided into sixty parts on the 
outside; but these are to express minutes of space. Every 
tenth is marked with large figures, but every single one is also 
denoted with its proper figure, in a smaller character. The 
degrees are shewn in a square opening under the centre, and 
change backwards and forwards as the telescope rises or falls. 
This piece may be made to shew polar distance, zenith dis- 
