In replying to these allegations, I shall first show, that, granting the dependence which M. Le Monier supposes may be placed on observations of the variation made at sea, he has stated the quantity of the variation observed on board the Resolution, very erroneously.
Secondly, I shall prove, beyond contradiction, that observations of the variation, made at sea, cannot be depended on for the purposes to which M. Le Monier has applied them.
And, lastly, that no material error had crept into M. Bouvet’s reckoning; but that if any error did exist, it must have been of a contrary nature to that which M. Le Monier supposes.
That M. Le Monier has not given altogether a true representation of the matter, will appear from hence. On the 16th of February, at noon[[27]], the Resolution was in latitude 54° 311⁄2ʹ south, which is sufficiently near the parallel of 54° south, to see high land, the northern extremity of which lies to the southward of that parallel; and at that time we were in 6° east of Greenwich, or 233⁄4° east of the island of Ferro: that is, 43⁄4° less than is assigned for our situation by M. Le Monier. On the evening of the same day, the ship being in latitude 54° 24ʹ, and longitude 6° 30ʹ, or 241⁄4° east of Ferro, the variation was no more than 12° 7ʹ west, which also is near a degree and a half less then M. Le Monier says it was, when we first arrived in a proper parallel for seeing Cape Circumcision. It is true, the next morning, in latitude 54° 211⁄2ʹ south, longitude 8° 6ʹ east, we had 13° 42ʹ west variation; but this was after we had run more than two degrees within sight of the parallel of 54° south. It is, moreover, highly probable, that both these variations were too great; for, on the 17th, in the evening, latitude 54° 25ʹ south, and longitude 9° 20ʹ east; that is, 11⁄4° more to the eastward, and after we had run 31⁄3° on the parallel we were then on, the variation was no more than 13° 16ʹ west. It is also worthy of remark, that on the 14th, in the evening, latitude 56° 141⁄2ʹ south, and longitude 4° 50ʹ east, which is but 1° 10ʹ to the westward of the point, where the Resolution came first into a proper situation to see land, situated in the parallel of 54° south, the variation observed was no more than 6° 50ʹ west. And we may further add, that on the 1st of March, 1774, the Adventure had no more than 123⁄4° west variation, though she was then considerably both to the northward and eastward of our situation on the 17th of February in the morning, on both which accounts the variation ought to have been greater, instead of a whole degree less. From all these circumstances, there can be little doubt but that the two variations, observed by us on the 16th and 17th of February, were too great; or that the variation, at the point where the Resolution first came sufficiently near the parallel of 54° south, to see land, the northern extremity of which is situated in that parallel, could not be more than 111⁄2° west, instead of 131⁄2°, as M. le Monier has represented it.
Under this head of enquiry, I may also observe, that, although the Resolution was too much to the southward of the parallel of 54° south, when she crossed the meridian which is 211⁄2° to the eastward of Ferro, that is, 33⁄4° east of Greenwich, the longitude which M. Le Monier assigns for Cape Circumcision, to see if it had been in that situation, yet her consort, the Adventure, was for several degrees on each side of that meridian; and especially when she had 101⁄2° of west variation, full as near to the parallel of 54° south as M. Bouvet was to the land when he saw it[[28]]; and on the day that she actually passed that meridian, had fine clear weather.[[29]] Hence, therefore, granting M. Le Monier his own arguments, which, however, I have proved to be erroneous, and that observations made at sea, for the variation of the compass, may be depended on for the purpose of finding the longitude, it is utterly impossible that both the Resolution and Adventure could have passed Cape Circumcision without seeing it. But I shall now show, that these observations are liable to a much greater error than the whole quantity, so vigorously insisted on by this gentleman.
I will not here run the risk of incurring M. Le Monier’s displeasure, by calling the accuracy of M. Bouvet’s observations in question; but will admit every thing that he himself can think due to the instruments and observations of that deserving navigator. It is enough for my argument, and it is but too evident from the observations themselves, that ours were by no means capable of determining the variation to so small a quantity as that which M. Le Monier rests his whole cause upon; and if so, his arguments, which depend wholly on a supposition, that not only they, but M. Bouvet’s also, were capable of determining it with the utmost exactness, must fall to the ground.
1st, It appears, from various instances, that the variations observed by the same compass would differ 3° to 5°, 6°, and sometimes even 10°, from no other cause whatever, but putting the ship’s head a contrary way.[[30]]
2d, That the same compass, in the same situation in every respect, within a few miles, but at two different times of the same day, would give variations differing from one another, 3°, 4°, 5°, 6°, and even 7°[[31]].
3d, That the same compass, on the same day, and in the hands of the same observer, will give variations differing from one another by 5°, on board the same ship, when under sail, and when at anchor in a road-stead.[[32]]
4th, Compasses made by the same artists at the same time and place, but on board different ships, differed 3°, 4°, and even 5° in the variation.[[33]]