SS 
andi Method of difcovering the Loxgitude. 38 
conclufions were rendered as uncertain as the variation itfelf, 
of which (it had fo happened from the method they purfued) 
they knew little more than that it did change. Had the 
prefent theory occurred to them, it would have received 
chearful and favourable countenance and encouragement, 
and it and its utility been foon and effectually eftablithed. 
Had the indefatigable and famous Dr. Halley in parti- 
cular known shaveus when he undertook two voyages (long 
afier the inclination of the magnetic needle was difco- 
vered, nanxly, in 1698 and 1699) for the exprefs purpofe 
of endeavouring to form a magnetic theory by means of a 
multitude of obfervations of the magnetic variation, as a pre- 
paratory ftep, and in order to determine the longitude there- 
by, or, as his inftructions run, “ to feek, by obfervation, the 
rule of the variation of the compafs ;”’? the obfervations of 
magnetic dip here recommended, and which he would have 
made at leaft along the weftern line before pointed out, toge- 
ther with’ the obfervations of the variation of the compat: 
which he did obferve, in all likelihood, if made with inftru- 
ments capable of affording the requifite accuracy, would have 
foon fuggefted, to his. penetrating and fruitful genius, the 
correct object of his refearch. 
It may be farther proper to remark, that the obfervations 
of the magnetic inclination herein ftated, and made at Lon- 
don, do of themfelyes alone eftablifh another important 
conclufion, likewife quite different from the general opinion 
of the Learned, which fuppofes that the magnetic variation 
affects (z. e. increafes or diminifhes) the magnetic inclina- 
tion; for, from the time in which the magnetic inclination 
was firft obferved at London to the prefent time, the varia- 
tion has fhifted upwards of 36° without affecting the inclin. 
ation; and therefore, after knowing the refult of fo very great 
a change in the magnetic variation without having affected 
the inclination, there can be no reafon to conclude that it 
can in any refpeét be affected thereby: a circumftance that 
renders the theory here pointed out {till more fimple and 
beautiful. 
Thus, by new and interefting application and teftimony 
of unerring fcience, praclical obfervation, and fair conclufion, 
3D 3 I have, 
