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

OBSEllVATIOXS OF IXTENSITY. 519 interval, the results are found extremely accordant at several other stations at which observations Avere repeated at distant intervals. In the first of the subjoined tables are given the times of vibra- tion at Rio at the four periods referred to ; and the corresponding times as a dipping needle. In its three last columns are shewn, the number of days comprised in each interval, — the increase in the time of vibration owing to the loss of magnetism, — and the result- ing daily correction, on the supposition in each case of the loss having been uniform in the interval during which it occurred. The second table contains the corrected times of horizontal vibration at each of the South American stations, at the dates respectively inserted, — the corresponding times as a dipping needle, — the times of vibration as a dipping needle at Rio de Janeiro at the same dates, derived from the observations in the first table, — and the resulting intensity at each station relatively to unity at Rio. Thus far the results are derived from Captain King's observations, unmixed with those of any other observer : but in order to bring Captain King's series into connexion with the gene- ral body of results of other observers, the values of his intensities are expressed in the final column in terms of the scale in common use, in which the force at Paris = 1,348, and at Rio de Janeiro 0,884) ; the latter being the mean of four independent determi- nations by the following observers, namely, 1817 and 1820 Freycinet 0,890 ^ 1827 LUtke 0,886 [q^^^ 1830 Erman 0,879 / ' 1836 Fitz-Roy 0,878j Port St. Elena is not included in this table, as no dip was observed there, and the total intensity consequently cannot be computed. The three stations, Madeira, Teneriffe, and Port Praya, at which the cylinder was vibrated in the outward voyage, are also without dips observed by Captain King. The deficiency at Port Praya has been supplied from Captain Fitz-Roy 's observations and my own, both having been made at the same place at which Captain King's intensity was observed, — Captain Fitz-Roy's at a later, and mine at an earlier date. At Madeira also the dip has been supplied from my observations, which were made in the British consul's garden at Funchal, where Captain King's cylinder was vibrated. 1 have