Passing the line.
2. The 8th of January, in the afternoon, we passed the line between 27° and 28° of longitude.
Remark on the variations.
3. Since the 2nd of January we could no longer observe the variations; and I only reckoned them by the charts of William Mountain and James Obson. The 11th, at sun-set, we observed 3° 17′ of N. W. variation; and the 14th, in the morning, I observed again 10′ of N. W. variation with an azimuth-compass, the ship then being in 10° 30′ or 40′ S. latitude, and about 33° 20′ W. longitude, from Paris. Therefore it is certain, that, if my estimated longitude is exact, and I verified it as such at the land-fall[[11]], the line of no variation is still further advanced to the westward since the observation of Mountain and Obson; and it seems the progress of this line westward is pretty uniform. Indeed, upon the same degree of latitude, where Mountain and Obson found 12° or 13° of difference in the space of forty-four years, I have found a little more than 6° after an interval of 22 years. This progression deserves to be confirmed by a chain of observations. The discovery of the law by which these changes happen that are observed in the declination of the magnetic needle, besides furnishing us with a method of finding out the longitude at sea, might perhaps lead us to the causes of this variation, and perhaps even to that of the magnetic power.
Causes of the variations found in going to the Brasils.
4. About the line we have almost always observed very great variations on the north-side, though it is more common to observe them on the south-side. We had an opportunity of guessing at the cause of it, the 18th of January passing over a bank with young fish, which extended beyond the reach of our sight, from S. W. by[by] W. to N. E. by[by] E. upon a line of reddish white, about two fathoms broad. Our meeting with it, taught us that since some days the currents set in to the N. E. by[by] E. for all fish spawn upon the coasts, whence the currents detach the fry and carry them into the open sea. On observing these variations N. of which I have spoken, I did not infer from thence, that it was necessary there should be variations westward together with them; likewise the 29th of January, in the evening, when we saw land, I had calculated at noon that it was ten or twelve leagues off, which gave rise to the following observations.
It has long ago been a complaint among navigators, and still continues, that the charts, and especially those of M. Bellin, lay down the coasts of Brasil too much to the eastward. They ground this complaint upon their having got sight of these coasts in their several voyages, when they thought themselves at least eighty or a hundred leagues off. They add, that they have several times observed on these coasts, that the currents had carried them S. W. and they rather choose to tax the charts and astronomical observations as erroneous, than suspect their ships reckoning subject to mistakes.
Upon the like reasonings we might have concluded the contrary on our course to Rio de la Plata, if by chance we had not discovered the reason of the variations[variations] N. which we met with. It was evident that the bank with the fry of fish, that we met with the 29th, was subject to the direction of a current; and its distance from the coast proved, that the current had already existed several days. It was therefore the cause of constant errors in our course; and the currents which navigators have often found to set in to the S. W. on these shores, are subject to variations, and sometimes take contrary directions.
This observation being well confirmed, and our course being nearly S. W. were my authorities for correcting our mistakes as to the distances, making them agree with the observations of the latitude, and not to correct the points of the compass. By this method I got sight of the land, almost the same moment when I expected to see it by my calculation. Those amongst us, who always reckoned our course to the westward, according to the ship’s journals, being contented to correct the difference of latitude by the observations at noon, expected to be close to the shore, according to their calculation, long before we had so much as got sight of it: but can this give them reason to conclude, that the coast of the Brasils is much more westward than Mr. Bellin has laid it down?
Observations on the currents.