Narrative of the surveying voyages of his majesty's ships Adventure and Beagle (vol.1): between the years 1826 and 1836 : describing their examination of the southern shores of South America, and the Beagles's circumnavigation of the globe

510 MAGXETIC IXTEXSITY. and the general results deduced for each station. The cokimn entitled " Time " is that of 300 vibrations ; and the " Coi-rected Time " is the mean of these, corrected for the rate of the chrono- meter and the arc, and reduced to an average temperature of 60^. The dips are those observed by Captain Fitz-Roy; except at Port Famine, where, as Captain Fitz-Roy did not observe, it has been supplied from Captain King's observations ; and at Coquimbo, where, for the purpose of computing the intensity, it has been supplied by estimation from the other geographic positions on this coast, at which Captain Fitz-Roy observed the dip. In the column showing the time of vibration as a dipping-needle at Plymouth corresponding to the periods of observation at the several stations, the compensations have been introduced for the variation in the intensity of the cylinder, agreeably to what has been said above on that subject. The two final columns exhibit the values of the total magnetic intensity at the different stations derived from these obsei'- vations. In the first of the two columns, the values are given rela- tively to the force at Plymouth, considered as unity ; and in the second column, relatively to the force at Plymouth, expressed by 1.375 ; for the purpose of exhibiting Captain Fitz-Roy 's results in direct comparison with the determinations of continental observers, who have taken Paris as their basis, giving the force at Paris the arbitrary expression of 1.3482. I have taken the ratio of the force at Plymouth to that at Paris to be as 1.375 to 1.348, which I believe will prove a very near approximation ; it is that whicli results from Captain Fitz-Roy's observations at Plymouth, in October 1836 (page 17), and mine, at Tortington, in Sussex, in June 1837 (page 10) : the dip at Tortington, at the period in question being 68° 57', and the intensity, compared with Paris, through the medium of London, 1.368,