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X. Account of the Equatorial Instrument. By Sir George 
Shuckburgh, Bart. F. R. S. 
Read March 21, 1793. 
“ Juvat ire per altum 
“ Aera, et immenso spatiantem vivere ccelo ; 
“ Hoc sub pace vacat tantum.” Manilius, lib. i. 
(1.) Before I enter upon the description of the instrument 
which I propose particularly to describe, it may not be im- 
proper to say something of the equatorial in general, and of 
such instruments as, having been made upon a similar prin- 
ciple, have been used by different astronomers, in different 
ages. 
The first account, that I meet with, of an astronomical in- 
strument that bears any resemblance to it, is to be found in 
Ptolemy, (lib. 5 of his Almagest) wherewith, he tells us, 
he determined the distance between the two tropics. This 
instrument is described under the name atrrpoXo'MiKOv opyuvov, 
and appears to have consisted of two circles, placed at right 
angles to each other, one representing the meridian or sol- 
sticial colure, and the other the zodiac ; the former turning 
upon an axis, placed parallel to the axis of the earth, being 
elevated to the latitude of the place, and the other turning 
within it on two centres, removed 23-t 0 from the former axis ; 
and was in truth not very unlike the common ring dial, only 
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