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A 'Letter ofMr.Dc la Hire of the Royal Academy of 
the Sciences at Paris^ concerning a new fort of Mag" 
netical Compafs^ mth fei^eral curious Magnetical 
^Experiments. 
YOU know Sr. that there is nothing which creates 
fb much trouble in long Voiages on the Sea, as the 
Variation of the Magneticall Needle, both becaufe this 
Variation is different in differing places, and becaufe in 
the fame place it changes confiderably in proceCs of time. 
It feems that if wee had exaft Obfervations of the irregu- 
larities of this Variation, made all over the Earth, and at 
a confiderable interval of time, one might difcover fome 
Period of this Motion, and eftaUifh a Syftem which 
might be of great ufe in Navigation. But feeing our old- 
eft Obiervations were made but about a hundred Years 
fmce, and in fome particular places only, they only ferve 
to let us know, that if there be a regular Motion, it muft 
needs be very flow : So that we can conclude nothing 
certain for the time to come from all that has been hither- 
to Obferved. This is not becaufe of any difficulty that 
there is in afcertaining this variation by Obfervation, fince 
it is found to Change but few Minutes in a Year ; but 
too much reliance muft not be upon the Obfervations of 
Pilotes, by realdn of the groft Errors which it is not eafy 
for them to prevent, for it often happens that near the 
place where the CompaJs is, tljere is much Iron, which 
draws the Needle, and caufes itto fliew a point on the 
Horizon much different from what it would, were it far- 
ther from the Iron ; wliich makes it be thought that there 
is a confiderable Variaiion where perhaps there is none 
