INTRODUCTION. 
XXV 
proportion of fea actually failed through, leaves no fuf- 
ficient fpace for the correfponding mafs of land, which, 
on 
6th, Different compafles, at the fame time, on board the fame fhip, and in every re~ 
fpedt under the fame circumftances, will give variations differing from one another, 3°, 
4 0 , 5 0 , and 6° *. 
Thefe differences, feveral of which happened very near the place in queftion, are all of 
them at leaft equal to, moll of them much greater, and fome of them double that which 
M. Le Monier founds his argument on, even according to his own account of it, which I 
have already fhewn is by no means admiffible; and, therefore, totally invalidate it. To 
allege that the inftruments 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 
inftruments and obfervers which M. Le Monier ’s 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 affert, and fewer ftill found weak enough to 
believe), it will avail nothing to the point in difpute, which muft evidently fall to the 
ground, if the obfervations made for finding the variation in Captain Cook’s voyage are 
not fufficient to fupport it. What then muft become of it, if M. Bouvet’s obfervations, 
of this kind, were liable to an equal, or a greater error ? which, without any reafonable 
eaufe for offence, we might fuppofe they were. 
It is not neceffary to account for thefe differences in the obferved variations in this 
place, nor yet to point out the reafons why fuch anomalies have not been noticed in ob¬ 
fervations of this kind before. I fnall, however, remark, that I have hinted at fome of 
the caufes in my introduction to the 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 considered the 
principles on which the inftruments are conftruCted, and the maimer in which they are 
fabricated. Nor is it at all furprizing, that the errors to which the inftruments and ob¬ 
fervations of this kind are liable, fhould not have been difcovered before, fince no navi¬ 
gators before us ever gave the fame opportunity, by multiplying their obfervations, and 
making them under fuch a variety of circumftances as we did. 
Having now fully fhewn, that the circumftances, brought forward by M. Le Monies 
in fupport of his argument, are neither fuch as can be depended on, nor yet fairly repre- 
fented 
* 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, Auguft 18, 
l 77 &> p« 180. October j> and 14.* p. 189, and 190. December 12, p, ibid, January 
2 4 > r ? 77 ’P- ! 9 2 - March 10, p. 193. July 9, and 17, 1779, p. 209. January 16, 1780, 
212. March 24, p. 213. and May 19, p. 214. 
Vol. I. d * 
