[35°] 
It remains only to explain after what manner tliefe cir- 
cular Needles may be touched a new, when it is perceiv- 
ed that they have loft their firft Vigour. Aecordmg to 
this Hypothefis, it is evident that if the Circle be not 
touched in the point that anfwers to that of the Stone, with 
regard to its Variation, the little Fleurde Lis which mark- 
ed the true North, may decline a little from it ; and the 
difficulty of finding the correfponding points on the Ring 
and the Stone, would caufe that the touch of the Circle 
could not be refrefhed, without taking great care and firft 
obferving the Meridian line. But to avoid all thefe diffi* 
Culties, you need only apply the Poles of the Stone to the 
Ring; and the Ring, which is fufpended upon its pivot, 
will turn C) as the Point anfwering to the Pole of the Ver- 
tue of the Stone which is applyed to it, will come as near 
to it as poffible : In fo much that without touching the 
One or the other, the Ring will not fail to receive very 
much force. The fame may be done at the oppofite Pole. 
I doubt not but you are curious enough to fee if the 
poles do change in the Terrella,\vh^n you fhall meet with 
One fit for this Experiment. There might feveral other 
things be noted upon this Subju9:,and it were to be willied 
that fbme other particular obfervations might be made as 
opportunity fhall offer ; but unlefs curiofities of this na- 
ture fall into the hands of fuch as have a great love for the 
advancement of the Sciences, it is not to be hoped that 
we fhall have any certain information in a matter fb nice, 
Paris 
April. 26. i6Sy^ 
This Letter having been produced and Read before 
the Royal Society ; it was Ordered that the Terrella^ 
whicli has been in tlieir Repofitory thefe 25 Years, 
the 
