F -xeg jj 
Ocroser 17881 publifhed a fmall treatife on the calculation 
of Areas arithmetically, in which I mentioned the variation found 
in September, which was 26 degrees 50 minutes wefterly, and 
gave a general rule for finding it at any time, as alfo a pro- 
blem for reftoring loft or defaced boundaries, by the varia- 
tion, as follows, viz. 
Havine the notes of a furvey recently taken (by the needle) to 
find what the bearing of any ftationary line was any number of 
years paft, fince the year 1657. 
R U L E: 
MuuripLy 12 minutes 20 feconds by the years elapfed, and 
add the produét to the prefent bearing, the fum will be the 
bearing required, and vice ver/a. 
I come now to the proof of my affertion, viz. that the change 
of the horizontal needle is uniform at Dublin. We have fun- 
dry authentic records to fhew that the variation was eafterly 
prior to the year 1657, and that in 1657 there was no variation 
in thefe kingdoms. With refpect to the former, Mr. Burrows, 
Mr. Gunter, and Mr. Gellibrand are fufficient evidence ; the firft, 
in 1580, obferved it to be 11. degrees 15 minutes caft of the meri- 
dian of Greenwich ; the fecond, in 1622, fix degrees eafterly ; and 
the laft, in 1634, four degrees five minutes eafterly. In the year 
1657 the magnetic and true meridians coincided in Ireland 
according to the Down Survey, or in other words, there was- 
no variation, 
In= 
