' (1580) 
look through them. Thefe Arms ought to be long 
enough for the Plumb-lines to reach the Polar- Star, on 
ene fides and the Sun at his greateft height, on the 
other fide, when you look through either of the Sights. 
The Plumb-lines therefore are Tangents to their oppofite 
Sights, and their lengths may be found by a Table of 
natural Tangents, and making the diftance of the two 
Sights Radius. Thus in the Latitude of if the 
Inftrument be two feet from Sight to Sight, the Southern 
Plijmb-iine hath need to be near 4 feet, and the Northern 
Plumb-line about 2 feet 10 inches. On the tops oi 
thefe two Arms place two fmall crofs pieces D. E. & D. E. 
to turn with a Joynt at D. which Crofi-pieces are to 
hold the Plumb-lines E. F. and E. F. and to turn off and 
on, fo as to bring the Plumb lines to the Sights exaftly. 
Place this Inftrument on a Pedeftal H. to turn round on 
it ftiffly at the Pin G. 
Your Inftrument being thus prepared, the way to 
Set and Vfe it is thus, Plant it in a convtni&nt place, 
where the Polar-Star may be feen by Night, and the Sun 
by Day. When that Star is on the Meridian, is the 
time to fet this Inftrument, which is thus to be done 5 
viz. Through the Sight with the larger Hole c. d. look 
at the Pole-Star, and turn the whole Inftrument about, 
until you fee the oppofite Plumb-line nicely to interfeft 
the Pole Star. Or when you have brought the Plumb- 
line near thqStar, you may more eafily bring the Plumb* 
line to interfeft, by moving the Sight c, d, backward or 
forward, at the Joynt I, inftead ef moving the whole In- 
ftrument. And that you may more eafily fee the Pole^" 
Star thro the Sight, let the Plumb-line be a very fine Cats- 
gut ftring or Horfe-hair, &c. And if it be white, or fome 
fuch light colcur^jit will be the better feen, with the help 
of a Candle ftiining on it by Nip.ht^ which is neceffary. 
The Sight and oppofite Plumb-line being thus fet 
10 a direft line with the Polar-Star on the Meridian, it is 
manifeft, 
