SUPPLEMENT. 
35 
and some very unexpected magnetieal phenomena have been 
discovered. That instrument was in use for some years before 
it was known that the needle had any deviation from the true 
polar direction. About the middle of the sixteenth century this 
began to be suspected, and observations which were made soon 
afterwards, proved that in England and its vicinity, it was east¬ 
erly. This easterly variation decreased until about the year 
1658 or 1660, when the direction of the needle corresponded 
with the meridian. After that time it became westerly, and 
continued gradually for a long period of time to increase in 
quantity. In the course of successive observations, it was found 
to differ in different parts of the world. Hence it became 
absolutely necessary, both on this account and because of the 
gradual alterations to which it was subject, that mariners should 
be furnished with the means of daily ascertaining in every 
situation, the quantity of error or variation of the compass, in 
order to correct the courses to be steered, and the bearing of 
objects seen. In ascertaining the quantity of this variation by 
the well-known method, the result was, till within a few years 
past, generally believed to be correct, or at least not subject to 
much error. Differences in these results were, however, at 
length observed by modern navigators, particularly by Mr. 
Wales, the astronomer, who accompanied Capt. Cook in his 
third voyage, these differences being from 3° to 6°, and even 
10° with the ship’s head in contrary directions ; under various 
other circumstances mentioned in the introduction to Cook's 
voyage, they were from 3 ° to 7 °. 
It was reserved, however, for that able and scientific naviga¬ 
tor, the late Captain Flinders,to elucidate this interesting fact; 
to explain the probable, and till then the unsuspected cause of 
this aberration of the needle, to draw conclusions and to lay down 
a rule for correcting the error of variation, occasioned by chang¬ 
ing the ship’s head, which under the circumstances, and within 
the limits of his observation and experience, were probably 
legitimate and correct. But tho principle on which this rule is 
founded, will not be found applicable to every circumstance and 
