[ S 68 ] • 
So is the Difference of Longitudes on A , or the 
Difference between the logarithmic Tangents of 
the Half Co-latitudes of two Places 
To the Difference of Longitudes on B, or the me- 
ridional Difference of Latitudes of thofe Places. 
And hence arife the Rules which arc given in nau- 
tical Works, for finding the meridional Parts by a 
Table of common logarithmic Tangents. 
This curious Difcovery of Dr. Halley’ s, joined to 
that excellent Thought of his, of delineating the Lines, 
fhewing the Variation of the Compafs, on the nautical 
Chart, arc fome of the very few ufeful Additions 
made to the Art of Navigation within the laft i yo 
Years: For if, befide thefe, we except the Labours 
of that ingenious Artift Mr. Richard Norwood, who 
improved the Art by adding to it the Manner of 
failing in a Current, and by finding the Meafure of 
a Degree on a great Circle, the Theory of Naviga- 
tion will be found nearly in the fame State in which 
it was left by that eminent Mathematician Mr. 
Edward IVnght ; who, about the Year 1600, pub- 
lifhed the Principles on which the true nautical Art 
is founded ; and fhewed, what does not appear to 
have been known before, how to cfiimate a Ship's 
true Place at Sea, as well in Longitude as in Lati- 
tude, by the Ufe of a Table of meridional Parts, 
firft made by himfelf, and conftruded by the con- 
ftant Addition of the Secants, and which differs al- 
moft ini'eniibly from fuch a Table made on Dr. Hal - 
Ay’s Principles, contained in the preceding Articles. 
Ifhall conclude this Difcourfe with anArticle, which, 
altho’ it be fomewhat foreign to the preceding Sub- 
fed, yet, as it was difeover’d while 1 was contem- 
plating 
I 
