r 8?8 ] 
waters ; the continuation of which, from fea to fea, 
will be thereby confpicuous, and we fhall be enabled 
to judge better of their nature, properties and caufes; 
and, if the fame can be extended over all the parts 
of the known world, the eye will be prefented, at 
one view, with the different degrees of attraction, 
with which all the parts of this great magnet are 
endued, at the time when fuch lines are drawn j This 
the writers would have attempted to have done, in 
the year 1 744., if they could have procured a fuffi- 
cient number of obfervations for that purpofej but 
although they frequently advertifed their requeft, in 
the public papers, no affiftance was thereby ob- 
tained. 
As the writers have by experience found, that the 
proper period for re-examining the date of the varia- 
tion is now at hand, without which the above-men- 
tioned valuable advantages of the Chart will be loft 
to the mariner ; they have determined to coiled and 
compare all the obfervations that can be procured 
by them, in the fpace of a year from this time, or fo 
long after as the return of the Ead-India fhips then 
next following j if fuch delay fhould become necef- 
fary, by the arifing of any doubt in confequence of 
fuch comparifonj and then to publifh the refult of 
their procefs, in fuch a manner as fhall feem mod; 
convenient. 
Several of the learned and ingenious have endea- 
voured to account for this phenomenon of the varia- 
tion of the magnetic-needle, and the continual mu- 
tation thereof ; whence different methods of com- 
putation have been propofed, whereby they have en- 
deavoured to determine what the quantity of the 
2 variation 
