r ‘rry 4 
Jiving cannot point out a certain land-mark to trace them by ; 
but while an ancient map exifts, or the original Field Notes, if 
the magnetic meridian be drawn, or the variation exprefled, every 
veftige of the land can be diféovered, and its boundaries defined 
with the greateft eafe. But this is only a fecondary confidera- 
tion: The firft and moft material of all is that of laying a foun- 
dation for a difcovery of THE LONGITUD= BY SPA OR LAND 3 
for certainly, were the magnetic laws univerfally known, the Lon- 
citupe could be more readily afcertaned by THEM, than by any 
other means. At Dublin we are pretty certain the annual change is 
iz minutes 20 feconds nearly, and when we know what it is in 
various other places to as great a certainty, we may expect to be 
“ put in poffeffion of fuch a fyftem * as will account for or foretel 
«© what will be the future movements of this wonderful pheno- 
“ menon, the magnetic influence,” all the world over. 
THO’ HARDING. 
Trinity Place, Dublin, 
April 23, 1791. 
N. B. I sHavy offer my obfervations on the Libration of the 
Needle, or what is commonly called the Diurnal Variation, dur- 
ing eight months, to the confideration of the Royal Irith Aca- 
demy, as foon as I can get time to adjuft them. 
Anfwers 
* Churchman, 
