C 417 ] 
upper Edge of the {hade Vane, to the 
Sun, and it will give what is required : 
The fame Rule is to be obferved in 
taking the Altitude of a Star. 
II. T he ‘Description and Ufe of an Appa- 
ratus added as an Improvement to Davis's 
Quadrant , conjijling of a Mercurial Level, 
for taking the Co-altitude of Sun or Star 
at Sea , without the ufual AJJijlance of the 
fenjihle Horizon , which frequently is ob- 
fcured . "By Charles Leigh, Gent. 
I Had the Honour fome time ago, [Nov. 3. 1737.] 
to communicate to this learned Society, an Inven- 
tion much upon the fame Nature and Principle with 
this 5 hnce which I have made fuch Alterations and Im- 
provements thereto, as have render'd it complete and 
perfect for theUfe intended, and have been confirmed 
by repeated Experiments, as well on board Ships, as on 
Shore. An Inftrument of this Nature we greatly want 
at Sea, and it would be a great Satisfaction to me, if any 
Thoughts and Inventions of mine fhould contribute 
to the removing of this grand Impediment, that fo 
frequently happens. 
To arrive to the utmoft Perfection in Navigation, 
three things are abfolutely requifite, viz. The Varia- 
tion, the Latitude, and the Longitude 5 which laft is, 
as yet, conceal'd from us. The two former indeed. 
