I must add what, in my opinion, is far from being the least recommendation of these materials, that the observations of the variations of the compass, which are laid down in the chart from these Spanish journals, tend greatly to complete the general system of the magnetic variation, of infinite importance to the commercial and sea-faring part of mankind. These observations were, though in vain, often publicly called for by our learned countryman, the late Dr Halley, and to his immortal reputation they confirm, as far as they extend, the wonderful hypothesis he had entertained on this head, and very nearly correspond, in their quantity, to the predictions he published about fifty years since, long before he was acquainted with any one observation made in those seas. The ascertaining the variation in that part of the world is just now of more than ordinary consequence, as the editors of a new variation chart, lately published, for want of proper information, have been misled by an erroneous analogy, and have even mistaken the very species of variation in that of the northern ocean; for they make it westerly where it is easterly, and have laid it down 12° or 13° different from its real quantity.
This much it has been thought necessary to premise, with regard to the hydrographical and geographical part of the ensuing work; which, it is hoped, the reader will find, on perusal, much ampler and more important than this slight sketch can well explain. But, as there are hereafter interspersed, occasionally, some accounts of Spanish transactions, and many observations relative to the dispositions of the American Spaniards, and to the condition of the countries bordering on the South Seas; and as herein I may appear to differ greatly from the opinions generally established; I think it behoves me particularly to recite the authorities I have been guided by in these matters, that I may not be censured as having given way, either to a thoughtless credulity on the one hand, or, what would be a much more criminal imputation, to a wilful and deliberate misrepresentation on the other.
Mr Anson, before he set sail upon this expedition, besides the printed journals to these parts, took care to furnish himself with the best manuscript accounts he could procure of all the Spanish settlements upon the coasts of Chili, Peru, and Mexico. These he carefully compared with the examinations of his prisoners, and the informations of several intelligent persons who fell into his hands in the South Seas. He had likewise the good fortune, in some of his captures, to possess himself of a great number of letters and papers of a public nature, many of them written by the viceroy of Peru to the viceroy of Santa Fee, to the presidents of Panama and Chili, to Don Blass de Lezo, admiral of the galleons, and to divers other persons in considerable employments; and in these letters there was usually inserted a recital of those they were intended to answer, so that they contained no small part of the correspondence between these officers, for some time previous to our arrival on the coast. We took, besides, many letters, sent from persons entrusted by the Spanish government, to their friends and correspondents, which were frequently filled with narrations of public business, and sometimes contained undisguised animadversions on the views and conduct of their superiors. From these materials those accounts of the Spanish affairs ore drawn, which may appear, at first sight, the most exceptionable. In particular, the history of the various casualties which befel Pizarro's squadron is, for the most part, composed from intercepted letters; though, indeed, the relation of the insurrection of Orellana and his followers is founded on rather a less disputable authority; for it was taken from the mouths of an English gentleman then on board Pizarro, who often conversed with Pizarro; and it was, upon enquiry, confirmed in its principal circumstances by others who were in the ship at the same time: so that the fact, however extraordinary, is, I conceive, not to be contested.