Sight of Cape Virgins.
On the 2d of December in the afternoon, we discovered Cape Virgins, and we found it bore S. about seven leagues distant. |Its position.| At noon I had observed 52° S. lat. and I was now in 52° 3′ 30″ of latitude, and in 71° 12′ 20″ of longitude west from Paris. This position of the ship, together with the bearing, places Cape Virgins in 52° 23′ of latitude, and in 71° 25′ 20″ of longitude west from Paris. As Cape Virgins is an interesting point in geography, I must give an account of the reasons which induced me to believe that the position I give is nearly exact.
Discussion upon the position given to Cape Virgin.
The 27th of November in the afternoon, the chevalier du Bouchage had observed eight distances of the moon from the sun, of which the mean result had given him the west longitude of the ship, in 65° 0′ 30″ for one hour, 43 min. 26 sec. of true time: M. Verron, on his part, had observed five distances, the result of which gave for our longitude, at the same instant, 64° 57′. The weather was fair, and extremely favourable for observations. The 29th at 3 hours 57 min. 35 sec. true time, M. Verron, by five observations of the distance of the moon from the sun, determined the ship’s west longitude, at 67° 49′ 30″.
Now, by following the longitude determined the 27th of November, taking the medium between the result of the observations of the chevalier du Bouchage and those of M. Verron, in order to fix the longitude of the ship, when we got sight of Cape Virgins, the longitude of that Cape will be 71° 29′ 42″ west from Paris. The observations made the 29th afternoon, likewise referred to the place of the ship, when we made the Cape, would give a result of 38′ 47″ more westward. But it seems to me that those of the 27th ought rather to be followed, though two days more remote, because they were made in a greater number by two observers, who did not communicate their observations to each other, and however did not differ more than 3′ 30″. They carry an appearance of probability which cannot well be objected to. Upon the whole, if a medium is to be taken between the observations of both days, the longitude of Cape Virgins will be 71° 49′ 5″, which differs only four leagues from the first determination, which answers within a league to that which the reckoning of my course gave me, and which I follow for this reason.
This longitude of Cape Virgin is more westerly by 42′ 20″ than that which M. Bellin places it in, and this is the same difference which appears in his position of Montevideo, of which we have given an account in the beginning of this chapter. Lord Anson’s chart assigns for the longitude of Cape Virgins, 72° west from London, which is near 75° west from Paris[[60]]; a much more considerable error, which he likewise commits at the mouth of the river Plata, and generally along the whole coast of Patagonia.
Digression upon the instruments proper for observing the longitude at sea.
The observations which we have now mentioned, have been made with the English octant. This method of determining the longitude, by means of the distances of the moon from the sun, or from the stars in the zodiac, has been known for several years. Mess. de la Caille and Daprès have particularly made use of it at sea, likewise employing Hadley’s octant. But as the degree of accuracy obtained by this method depends in a great measure upon the accuracy of the instrument with which you observe, it follows that M. Bouguer’s heliometer, if one could measure great angles with it, would be very fit for rectifying these observations of distances. The Abbé de la Caille probably has thought of that, because he got one made, which would measure arcs of six or seven degrees; and if in his works he does not speak of it as an instrument fit for observing at sea, it is because he foresaw the difficulty of using it on board a ship.
M. Verron brought on board with him an instrument called a megameter, which he has employed in the other voyages he made with M. de Charnieres, and which he has likewise made use of on this. This instrument appeared to be very little different from the heliometer of M. Bouguer, except that the screw by which the objectives move, being longer, it places them at a greater distance asunder, and by that means makes the instrument capable of measuring angles of ten degrees, which was the limit of M. Verron’s megameter. It is to be wished, that by lengthening the screw, we were able to augment its extension still more, it being confined in too narrow bounds to allow a frequent repetition, and even to make the observations exact; but the laws of dioptrics limit the removing of the objectives. It is likewise necessary to remedy the difficulty which the Abbé de la Caille foresaw, I mean, that which arises from the element on which the observation must be made. In general, it seems that the reflecting quadrant of Hadley would be preferable, if it were equally accurate.
Difficulties on entering the straits.