[ 876 ] 
of this variation is in a perpetual date of flu&uation, 
in (perhaps) every part of the world, it had been fo 
much changed in the i'pace of forty years, or there- 
abouts, that (when the writers of this paper endea- 
voured about the year 1744, t0 draw thereon, 
other lines to anfwer the purpofes above-mentioned) 
they found that thofe laid down by Do&or Halley 
were grown intirely ufelefs ; and that a fydem of 
fuch lines, or fomething analogous thereto, fhould 
be performed once in every ten or twelve years at 
lead, in order to anfwer the purpofes intended, by 
that fagacious gentleman. 
In the recondrudlion of them, the writers received 
the aflidance of the commiidoners of the navy, and 
of the directors of the Ead-lndia and African com- 
panies, having leave to perufe the journals of thofe 
mariners, which were under the direction of each 
refpe&ive body ; From thefe, and a few private com- 
munications, they were enabled to draw the proper 
lines over the mod frequented feas, and to make 
fome attempts toward doing the fame in thofe lead 
fp i A copy of the Chart, thus again rendered ufeful, 
they presented to this Royal Society, with an account 
of the methods ufed in performing the fame. 
Although the mod beneficial ufe of thefe lines be- 
longs to the fea, yet if they could be extended over 
the land likewife, the advantages arifing would more 
than compenfate the trouble, as will appear by taking 
a fhort view of each. 
And fird, the ufe of thefe lines at fea may be con- 
fidered either as common to the art of navigating in 
all large bodies of water j or as particular in fome 
fuch : the general ufe being that of deering the true 
courfe 
