C 415 3 
perpendicularly two glafs Tubes Ehzn&dh, fourln* 
ches long, with brafs Ferrels on the Tops. 
On the central Pin, which is fix'd in the Index, is 
alfo fix'd the brafs horizontal Vane Ez obliquely, 
in which there is a Hole for the central glafs Tube 
Eh , to come through three fourths of its Length, 
clofe to which, and from the common Centre, comes 
a white fine Thread, the End being fix'd in the Vane 
Ezb and in the fame manner is a Thread fix'd clofe to 
the glafs Tube dh. 
Directions to prepare , and obferve by this 
Injtrument. 
To prepare this Inftrument for Obfervation, you 
muft pour Water (for that is always to be had) into 
the Tube Eh , till its little Surface rifes to the central 
Thread 5 then to keep it fix'd there, fhut the Slide or 
Stop that is fix'd on the Top of the central Tube, 
and there it will continue 5 then you may at Plea- 
fure pour or drop Water into the Tube dh , till its Sur- 
face alfo rifes to the Thread fix'd there 5 and if too 
much Water is dropp'd in, dip in a Wire with a fmall 
bit of Spunge or Cotton fix'd to the End, till you 
exaftly trim your Tubes 5 for in this lies the greateft 
Nicety and Exa&nefs, to trim your Surfaces true to 
the Threads. 
This being done, you are prepared for Obferva- 
tion ; and placing yourfelf conveniently, where there 
is the lead Motion, fit down on a Stool or the Deck, 
and having the Quadrant in its proper Pofition on 
your Lap, open the Slide on the Top of the Tub zEh* 
that the Water may have its natural Tendency, which 
Hhh 2 will 
