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XLI. A Letter to the Right Honourable the 
Earl of Macclesfield, Prefdent , the Coun- 
cil, and Fellows, of the Royal Society, 
concerning the V ariation of the Magnetic 
Needle \ with a Sett of Tables annexed-, 
which exhibit the Rejult of upwards of 
Fifty Thoufand Obfervations , in Six perio- 
dic Reviews, from the Year 1700 to the 
Year 1756, both inclufive ; and are ad- 
apted to every Five Degrees of Latitude 
a 7 id Lo?igitude in the more frequented 
Oceans. By William Mountaine and 
James Dcdfon, Fellows of the Royal So- 
ciety . 
Dated London, Nov. 9th. 175 7. 
SIRS, 
Read Nov. 10. N the 20th of March 1755, we 
V s _ > / presented an addrefs to this illus- 
trious Body, intituled, “ An Attempt to point out, 
<c in a concife manner, the Advantages which would 
“ accrue from a periodic Review of the Variation of 
“ the Magnetic Needle, throughout the known World ; 
<c requefting contributions thereto, by communicat- 
“ ing fuch obfervations concerning it, as had then 
“ been lately made, or could be procured from cor- 
“ refpondents in foreign parts.” 
This addrefs was read at the fame time, and af- 
terwards honoured with a place in the Tran factions, 
vol. xlviii. part ii. for 1754.: which favour we now 
acknowlege in the mod grateful manner ; and, pur- 
Vol. 50. U u fua«t 
