[ 2I 7 ] 
fight of land, where the obfervations arc afcertained 
to the fpot. Hence, if mine obferved about the 
year 1756, and thofe of Mr. Rofs’s, were both 
near the truth, at the refpeCtive times when they 
were taken, I know not how to account for this 
confiderable encreafe, unlefs thofe late extraordinary 
convulfions, in the bowels of the earth, upon thofe 
feveral coafts, may be found, by further experiments, 
to have there influenced the directions of the mag- 
netic needle. 
When thefe obfervations firft came to my hands, 
I intended to employ them, among others, in the 
conftrudtion of a variation chart, at fome proper pe- 
riod of time ; but, by reafon of a long and fevere 
indifpofition, I am obliged to fufpend thofe my in- 
tentions. As, therefore, they appear to be made by 
a judicious and accurate obferver, and, as Mr. 
Rofs remarks, upon fome local attractions in the 
Weft-Indies, mentioned at the clofe of his letter 
(which, without doubt, obtains in many other 
places) may induce fome able philofopher, fkilled in 
thefe matters, to inveftigate the caufe of thofe 
phenomena ; I beg leave to addrefs the whole to 
your lordlhip, to be difpofed of, as you fliall think 
proper. I am, 
My Lord, 
Your Lordlhip’s moft obedient fervant. 
Gainsford-ftreet, South- 
wark, May 18, 1766. 
W. Mountaine. 
Vol. LVI. 
F f 
Extract 
