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Terrestrial Magnetism . 
by them, though numerous, and by skilful observers, 
afforded but approximations to the truth ; yet they served 
the good purposes of showing their importance to safe 
navigation, and of directing attention to the necessity of 
procuring more information as to the amount of the 
variation of the needle in different places. Accordingly 
there exist several good accounts of the proceedings in 
this respect, which led to the proof that the variation of 
the compass is not constant at the same place, but is 
continually undergoing a progressive, though slow or 
gradual, change. 
Thus, in the beginning of the 17th century, the needle 
in London was inclined a few degrees to the eastward of 
the true north ; about 1660 it pointed to the true north ; 
and after that the variation became westerly, and has 
continued so until the present time. 
The line of no variation has been progressing slowly 
in a westerly direction, and has now passed over to North 
America. In London the variation now amounts to 
about 24° W. It was also subsequently ascertained that 
the dip was undergoing similar changes, although not to 
so great an amount: in 1680, the dip in London was 
ascertained to be 73° 30'—it is now about 69° 20'. 
This appears to be the amount of what was known on 
the subject until the latter end of the 18tli and the 
beginning of the 19th century, when the attention of 
those great navigators, Cook and Flinders, was attracted 
by the fact, that the mariner’s compass at sea was liable 
to error from the quantity of iron contained in various 
parts of a ship, and that a compass would not point the 
same w*ay with the ship’s head in various directions, or 
when placed in different parts of the ship; a fact which 
might be productive of most fatal consequences. The 
observations of Captain Flinders, which excited con¬ 
siderable attention at the time when they were made, are 
