C 216 ] 
Received June 6 , 1766 . 
XXV. A Letter fromMr.'W illiamMountayne, 
F. R. S. to the Right Honourable [ames 
Earl of Morton, P ref dent of the Royal 
Society , containing fome Obfervations on the 
V ariation of the Magnetic Needle , made 
on Board the Montagu Man of TV ar, in 
the Tears 1760, ijbi^and 1762, by Mr, 
David Rofs, Surgeon . 
My Lord, 
Read June 13,^ i > H E following tables I have compared 
J- with the Variation Chart, published 
in the year 1756, and do find that they agree pretty 
well in general, making allowance for the time 
elapfed * : it is true, that, in fome few places in the 
Atlantic Ocean, they differ; yet this may probably 
arife, as is often the cafe, from an error in the Mon- 
tagu’s fuppofed longitude, where fuch obfervations 
were made. But the greateft difference (a greater 
than fhould arife, I think, according to common 
courfe) appears upon the coaft of Portugal, Cape 
Saint Vincent, and about Gibraltar, near and within 
* By this expreflion, I do not mean that the variation under- 
goes a regular and uniform alteration, notwithftanding fuch ap- 
pearance, but rather fufpedt the contrary. 
fight 
