393 
and on the Dip of the magnetic Needle. 
stices have been noticed by M. Cassini, in his observations 
made at the Royal Observatory at Paris, between the years 
1783 and 1788, but the effect was considerably greater in his 
observations, than in those mentioned above ; his results 
however were, in my opinion, drawn from too few obser- 
vations, being from only 8 days observations about the times 
of the equinoxes and solstices, which differ considerably 
among themselves ; and experience teaches us, that mag- 
netical observations made for a period so limited are not 
sufficient for minute purposes : I have therefore, in the results 
here given, taken the mean of the observations made during 
the whole month in which the equinoxes and solstices fall," 
which appear to me likely to furnish results more satisfactory ; 
and all the foregoing observations are to be considered as the 
results or mean of a great many, by way of arriving at 
greater accuracy than could be obtained without ; this, how- 
ever, was found to be more necessary at some times than at 
others ; sometimes, the needle would be extremely consistent 
with itself, so as to return exactly to the same point, however 
often it might have been drawn aside ; at other times it varied 
2 or 3', sometimes 8, io', or even more; this uncertainty in 
the needle arises principally, I believe, from changes in the 
atmosphere, for, a change of wind, from any quarter to ano- 
ther, almost always produced a change in the needle from 
steady to unsteady, and vice versa , but it was generally more 
unsteady with an easterly wind, than when it blew from any 
other quarter, and most steady when the wind was south or 
south-westerly. An Aurora Borealis always produced consi- 
derable agitation of the needle. 
It has been mentioned in this Paper, that the annual increase 
3 E 2 
