[ 559 ] 
VI. A Letter from Mr . John Robertfon to 
the Prefident, contawing an Explanation 
of the late Dr. Halley V Demonflration of 
the Analogy of the Logarithmic Tangents 
to the Meridian Line, or Sum of the 
Secants. 
Rtad Nov. 2 2 . 
*7 S°- 
M 
SIR , 
Y Curiofity having lately 
led me 
to perufe feveral Books on the 
Art of Navigation, I was fomewhat furprifed not to 
find in any one of them a clear Explanation of that 
mod curious Paper in N p 219. of the Tkilofophical 
Tran fall ions, written by the excellent Mathematician 
Dr. Halley ; who, not intending to write for Begin- 
ners, as himfelf confelTes, has drawn his Conclu- 
fions in a manner, that feems to ftand in need of 
an Explanation, for the Generality of Readers: And 
as the maritime People arc not the beft acquainted 
with mathematical Knowledge, it might have been 
expeded, that fuch of the Writers on Navigation 
within the laft yo Years, who have undertaken to 
demonftrate the leveral Parts of their Subjed, would 
have removed the Difficulties in the Doctor’s Paper,, 
inftead of leaving them in the fame State in which 
they firft appeared. 
Dr Halley y in this Trad, feems to have had two 
chief Points in View ,* Firlt, To prove, that the 
Divtjions of the Meridian Line in a Mercators 
Chart 9 
