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VARIATION (continued)
Where the west variation, in the longitude 4O� E. is the same in both the above years; and in 1700 the west variation seemed to be regularly decreasing from longitude 50� E. to the longitude 100� E. but in 1756, we find the west variation decreasing so fast, that we have east variation in the longitude 80�, 85, and 90� E. and yet in the longitude 950 and 100� E. we have west variation again. Philosophical Transactions for the year 1757.To these remarks may be subjoined the following extracts from the Exposition du calcul astronomique, by M de la Lande.
At the royal observatory in Paris, a magnetical needle of four inches deviated from the N. 18� 10' towards the wet, on the 15th of February 1759: and on the 22d of April 1760, the same needle varied 18� 20'. It is indeed natural to conceive, that nothing can be precisely ascertained by ten minutes upon a circle whole diameter is only four inches. It is nevertheless sufficiently evident, that this variation continues to increase at Paris. In 1610 the needle declined 8� towards the eat, so that the variation has changed 26� 20' in the space of 150 years; and this appears particularly since 1740: for the same needle, which has always been used by M. Maraldi, is more than 3� advanced towards the west, beyond what it was at that. period; and this makes 9' in one year.
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© Derived from Thomas Cadell's new corrected edition, London: 1780, page 306, 2003
Prepared by Paul Turnbull
http://southseas.nla.gov.au/refs/falc/1444.html