INTRODUCTION. xxv 



prefervcd, though the proportion of fea aelually failed 

 through, leaves no fufficicnc fpace for the correfponding 



mafs 



6th, DifFerent compalTes^ at the fame time, on board the fame fliip, and in every 

 refpedl under the fame circumftances, will give variations differing from one another, 

 3°, 4% 5% and 6° *. 



Thefe differences, fcveral of wrhich happened very near the place in queflion, are all 

 of them at leafl: equal to, mofl of them much greater, and fome of them double that 

 which A'l. Le Monier founds his argument on, even according to his own account of it, 

 which I have already fliewn is by no means admifTible, and, therefore, totally invali- 

 date it. To allege that the inflruments made ufe of in Captain Cook's two voyages 

 were bad, or that the obfervers were not expert in the ufe of them, will anfwer no 

 purpofe : they are the inflruments and obfervers which M. Le Monier't, argument 

 muft reft on ; and, therefore, let thofe of the French, or any other navigator, have 

 been ever fo much better' than they|were (which few will be hardy enough to afTert, and 

 fewer flill found weak enough to believe), it will avail nothing to the point in difpute, 

 which mufl evidently fall to the ground, if the obfervations made for finding the varia- 

 tion in Captain Cook's voyage are not fufficient to fupport it. What then mufl be- 

 come of it, if M. BouveC'i obfervations, of this kind, were liable to an equal, or a 

 greater error ? which, without any reafonable caufe for offence, we might fuppofe 

 they were. 



It is not neceffary to account for thefe differences in the obferved variations in thts 

 place, nor yet to point out the reafons why fuch anomalies have not been noticed in 

 obfervations of this kind before. I fhall, however, remark, that I have hinted at 

 fome of the caufes in my introdu£lion to thd obfervations which were made in Captain 

 Cook's fecond voyage ; and many others will readily offer themfelves to perfons who 

 have had much practice in making thefe obfervations, and who have attentively con- 

 fidered the principles on which the inflruments are conflruiSed, and the manner in which 

 they are fabricated. Nor is it at all furprifmg, that the errors to which the inflru- 

 ments and obfervations of this kind are liable, fhould not have been difcovereJ before; 

 fmce no navigators before us ever gave the fame opportunity, by multiplying their 

 obfervations, and making them under fuch avariety of circumftances as we did. 



Having now fully fhewn, that the circumftances, brought forward by M. Le Mo- 

 nier^ in fupport of his argument, are neither fuch as can be depended on, nor yet 



fairly 



• Obfervations made in the fecond Voyage, February 2, 1773, p. 371- March 18, 

 p. 372. and January 24, 1774.. p. 37?. See alfo Obfervations made laft Voyage, 

 .Augull ]8, 1776, p. iSo. Odober 7, and 14, p. i8g, and 190. December 12, p. ibid. 

 January 24, 1777, p. 192. March 10, p. 193. July 9, and 17, 1779, p. 209. January 16, 

 i7fo, p. 21Z. March 24, p. 213. and May 19, p. 214. 



Vol. I, d 



