■[ 218 ] 
Extra hi of a Letter from Mr, David Rofs, Surgeon 
of His Majefly's Ship tloe Montagu, to Mr. 
William Mountaine, F. R. S. relating to the Va- 
riation of the Magnetic Needle j with a Sett of 
Obfervations made by bimfelf on Board the fa id 
Ship , during the Tears 1760, 1761, and 1762. 
E p e at e d obfervations of the variation of the 
magnetic needle, when the fpot they are taken 
upon can be tolerably well afcertained, are of great 
lervice to philofophy in general, but particularly to 
navigation, as in future ages they may ferve as a 
balls, to found its theory upon. 
Having had an opportunity of obferving, with all 
the care and accuracy in my power, the deflexion 
of the magnetic needle, by an azimuth compafs of 
Dr. Gowen Knight’s conllruddion, during a pallage 
to the Weft-India Iflands, a cruife there, and return 
to Britain ; and likevvife a cruife about the Straits of 
Gibraltar, I with pleafurc communicate them to 
you, as a perfon deferving the highefl: efteem of the 
commercial part of Britain, by your excellent new 
edition of the Variation Chart of the late celebrated 
Dr. Halley ; and I beg, Sir, your favourable ac- 
ceptance of them. 
I can allure you, I have endeavoured, as far as 
the thing would admit of, to get at the truth, having, 
at every oblervation, not trulled to a iingle azimuth. 
N' 6, in Virginia-Street, April 9, 1763. 
S I R, 
but 
