5th, The same compasses, on board the same ship, and within a few miles of the same situation, but at different times of our being there, gave variations differing by 4° and 5°, or upwards.[[34]]

6th, Different compasses, at the same time, on board the same ship, and in every respect under the same circumstances, will give variations differing from one another, 3°, 4°, 5°, and 6°.[[35]]

These differences, several of which happened very near the place in question, are all of them at least equal to, most of them much greater, and some of them double that which M. Le Monier founds his argument on, even according to his own account of it, which I have already shown is by no means admissible, and, therefore, totally invalidate it. To allege that the instruments made use of in Captain Cook’s two voyages were bad, or that the observers were not expert in the use of them, will answer no purpose: they are the instruments and observers which M. Le Monier’s argument must rest on; and, therefore, let those of the French, or any other navigator, have been ever so much better than they were (which few will be hardy enough to assert, and fewer still found weak enough to believe), it will avail nothing to the point in dispute, which must evidently fall to the ground, if the observations made for finding the variation in Captain Cook’s voyage are not sufficient to support it. What then must become of it, if M. Bouvet’s observations, of this kind, were liable to an equal, or a greater error? which, without any reasonable cause for offence, we might suppose they were.

It is not necessary to account for these differences in the observed variations in this place, nor yet to point out the reasons why such anomalies have not been noticed in observations of this kind before. I shall, however, remark, that I have hinted at some of the causes in my introduction to the observations which were made in Captain Cook’s second voyage; and many others will readily offer themselves to persons who have had much practice in making these observations, and who have attentively considered the principles on which the instruments are constructed, and the manner in which they are fabricated. Nor is it at all surprising, that the errors to which the instruments and observations of this kind are liable, should not have been discovered before, since no navigators before us ever gave the same opportunity, by multiplying their observations, and making them under such a variety of circumstances as we did.

Having now fully shown, that the circumstances, brought forward by M. Le Monier, in support of his argument, are neither such as can be depended on, nor yet fairly represented, I shall next attempt to demonstrate, that it is utterly improbable M. Bouvet could be out, in his account of longitude, so much as is here supposed, in the short run which had been made from the island of St. Catherine, the place they took their departure from: on the contrary, that there is sufficient reason to believe the error, of whatever magnitude it might be, was of a different nature from that contended for, and that the two ships, instead of being to the westward of their account of longitude, were actually to the eastward of it. For according to their journals, extracted from the archives of the French East-India Company, by M. D’ Apres, printed under his inspection, and published by Mr. Dalrymple, F. R. S. amongst other voyages made for the purpose of examining the southern parts of the Atlantic Ocean, the longitude, according to the Eagle’s run from St. Catherine’s, was, 26° 27ʹ, and according to the Mary’s, 26° 20ʹ east of Teneriff; that is, 9° 57ʹ, and 9° 50ʹ east of Greenwich, or 27° 43ʹ, and 27° 36ʹ east of Ferro. But the Mary, which went to the Cape of Good Hope, made 7° 13ʹ east longitude from the land in question, to that place. Consequently the Cape of Good Hope being in longitude 18° 23ʹ east of Greenwich, Cape Circumcision will be in 11° 10ʹ east of Greenwich, or 1° 20ʹ more to the eastward than the run by the same ship from the island of St. Catherine’s makes it. Again the Eagle made the difference of longitude between Cape Circumcision and the island of Rodrigues 49° 44ʹ; and by the observations of M. Pingre, this island is in 62° 50ʹ of east longitude from Greenwich: Cape Circumcision is therefore in 13° 6ʹ east of Greenwich, or 2° 9ʹ more to the eastward than by the Eagle’s run from St. Catherine’s. Hence, therefore, as the longitude of this land resulting from a comparison of that shown by each of the ships, on their making land at places where the longitude is exceedingly well determined, is greater than that which results from their run from St. Catherine’s, the longitude of which is not known with certainty within several degrees, we may infer, with great safety, that whatever the quantity of M. Bouvet’s error might be, when he is supposed to have seen Cape Circumcision, it must have been in defect, and not in excess, as M. Le Monier supposes it.

Christ’s Hospital,

April 20. 1784. W. WALES.

[27]. I here go by the dates in “The Original Astronomical Observations,” printed by order of the Board of Longitude; which, after the 14th of February, 1775, differ one day from Captain Cook’s date.

[28]. See The Original Astronomical Observations, p. 185., and Bouvet’s Voyage, published by Mr. Dalrymple, p. 4. and 11.

[29]. See the Observations p. 218.