( 457 ) 
The Latitude and Hour being given, the Varia- 
tion of the Compafs is eafijy known. 
• 
N. B. In order to draw Arches on the Globe ; 
rub fome black Lead powdered on a piece of Pa- 
per ; lay the Side which is black’d next the Globe, 
where you imagine the InterledUon of tfaeArches will 
be : Then draw them on the clean Side with the 
Point of the Beam-compafs, and they will appear on 
the Globe ^ and if the Globe is well vamilhed, they 
may be rubbed out with Bread, or wafhed out with 
Water. 
i 
As Altitudes at Sea are now readily taken, with 
great Exadtnefs, by the Quadrant invented by John 
Hadley , Efq; V. P. R. S. and as the faid Altitudes 
are the Principles on which the Operations above 
defcribed are founded ; the previous Ufe of that Qua- 
drant cannot but be of the utmoft Importance to 
thole who fliall have Occafion for this Inftmment, 
The Defcription and Ufe of this Inllrument was 
laid before the Royal Society Dec . p. 173 1 ; but as 
1 knew Mr. Reid was contriving one for the fame 
Purpole, I delay’d making mine Fublick. His Me- 
thod not yet appearing in Print, I have thought pro- 
per to communicate my own (elpecialiy as ’tis now 
improv’d) conceiving it may be of fome Advantage 
to Navigation* 
