Narrative of the surveying voyages of his majesty's ships Adventure and Beagle (vol.2- Appendix): between the years 1826 and 1836 : describing their examination of the southern shores of South America, and the Beagles's circumnavigation of the globe

S^6 APPENDIX. magnetic action in consequence of a ship's head being for a consider- able time towards the east, or west : yet this is but a conjecture. In the measures between Bahia and Rio de Janeiro, and in those between Rio de Janeiro and Cape Horn, there is no evidence of any permanent cause of error ; but the greater part of those measurements were made with the ship's head usually near the meridian. Were I to select three measurements which I thought less trust- worthy than others — I should decide on that from the Galapagos to Otaheite, from Otahelte to New Zealand, and from Hobart Town to King George Sound ; but I do not think that either one of these can be five seconds of time in error, according to regular computation, without supposing some unknovsm cause of error to exist. If each of the three were five seconds wrong, and each error lay in the same direction, still there would only be fifteen seconds out of thirty- two accounted for. Such a, supposition as this, however, that each of these three measurements is five seconds, or thereabouts, in error (referring only to error caused by known means) appears to be ex- tremely improbable, I would almost say impossible. It will naturally occur to the reader, that as error, undetected as to locality, exists, arbitrary correction must be made in order to reduce 24h. Om. 33s. to 24h. Otaheite has been selected as a point at which such a correction might be made vdth the least degree of inconvenience : to that place the longitudes in the accompanying tables are given as measured westward by Cape Horn, and eastward from Greenwich by the Cape of Good Hope ; and there, as the two portions of the chain overlap, a mean has been taken between the resulting longitudes. I will now recapitulate the principal measurements, and confront them with various other determinations. Limited space prevents my quoting many ; but I trust that enough will be given to show that some weight may be attached to at least a proportion of the results obtained by the Beagle's officers.