.380 Dr. Herschel’s Description of a 
to the great beam, towards the south, for instance, is hung a 
ring with a pulley to it ; while, at the same time, a similar 
ring and pulley is fastened to another post, near the opposite 
end of the beam, but situated towards the north. The ends of 
the rope are now returned through these pulleys, and with 
iron hooks, which are fastened to them, are hung in the loops 
at their respective ends of the middle 'beam. 
As soon as the ropes are sufficiently and equally stretched, 
the telescope will begin its horizontal motion, which may be 
continued as long as the same posts will be conveniently si- 
tuated. In order to go on with the motion, the ropes are to 
be slackened, and the rings being then hung upon the two next 
posts, we may continue at pleasure to turn the telescope to 
any . part of the heavens that may be required. The arrange- 
ment is represented upon a small scale in fig. 38. 
It should be noticed, that the ends of the rope must be 
equally stretched, for which reason a mark ought to be made 
in the place, which is to be thrown round the spoke of the 
wheel. The fastenings of the pulleys also, which are joined to 
the rings that are thrown over the posts, ought to have an ad- 
justment by links and hooks, to be either lengthened or short- 
ened at pleasure. 
With the assistance of the motions that have now been de- 
scribed, I have in the year 1789, many times taken up Saturn, 
2 or 3 hours before its meridian passage, and kept it in view 
with the greatest facility, till 2 or 3 hours after the passage ; a 
single person being able, very conveniently, to continue both 
the horizontal and vertical motions, at the command of the ob- 
server. In this however ought to be included an assisting third 
-motion, which I am in the next place to explain. 
