“Its circumference, I think, must be at least half a mile; as it took us above half an hour to walk round it, including every allowance for the bad road, and stopping a little. At its highest part, which is the south end, comparing it with a known object, it seems to equal the dome of St. Paul’s church. It is one uninterrupted mass of stone, if we except some fissures, or rather impressions, not above three or four feet deep, and a vein which runs across near its north end. It is of that sort of stone called, by mineralogists, Saxum conglutinatum, and consists chiefly of pieces of coarse quartz and glimmer, held together by a clayey cement. But the vein which crosses it, though of the same materials, is much compacter. This vein is not above a foot broad or thick; and its surface is cut into little squares or oblongs, disposed obliquely, which makes it look like the remains of some artificial work. But I could not observe whether it penetrated far into the large rock, or was only superficial. In descending, we found at its foot a very rich black mould; and on the sides of the hills, some trees of a considerable size, natives of the place, which are a species of olea.[[102]]
“In the morning on the 20th, we set out from the Pearl; and going a different road from that by which we came, passed through a country, wholly uncultivated, till we got to the Tyger hills, when some tolerable corn-fields appeared. At noon, we stopped in a hollow for refreshment; but, in walking about here, were plagued with a vast number of musquitoes or sand-flies, which were the first I saw in the country. In the afternoon we set out again, and in the evening arrived at the Cape Town, tired with the jolting waggon.”
On the 23d, we got on board the observatory, clock, &c. By a mean of the several results of the equal altitudes of the sun, taken with the astronomical quadrant, the astronomical clock was found to lose on sidereal time, 1ʹ 8ʺ, 368 each day. The pendulum was kept at the same length as at Greenwich, where the daily loss of the clock on sidereal time, was 4ʺ.
The watch, by the mean of the results of fifteen days’ observations, was found to be losing 2ʺ 261, on mean time, each day; which is 1ʺ 052 more than at Greenwich: and on the 21st, at noon, she was too slow for mean time by 1h 20ʹ 57ʺ 66. From this, 6ʹ 48ʺ, 956, is to be subtracted, for what she was too slow on the 11th of June at Greenwich, and her daily rate since; and the remainder, viz. 1h 14ʹ 08ʺ 704, or 18° 32ʹ 10ʺ, will be the longitude of the Cape Town by the watch. Its true longitude, as found by Messrs. Mason and Dixon, is 18° 23ʹ 15ʺ. As our observations were made about half a mile to the east of theirs, the error of the watch, in longitude, is no more than 8ʹ 25ʺ. Hence we have reason to conclude, that she had gone well all the way from England, and that the longitude, thus given, may be nearer the truth than any other.
If this be admitted, it will, in a great measure, enable me to find the direction and strength of the currents we met with on this passage from England. For, by comparing the latitude and longitude by dead reckoning, with those by observation and the watch, we shall from time to time, have, very accurately, the error of the ship’s reckoning, be the cause what it will. But as all imaginable care was taken in having and keeping the log, and every necessary allowance made for lee-way, heave of the sea, and other such circumstances, I cannot attribute those errors that did happen, to any other cause but currents; but more particularly when the error was constantly the same way, for several days successively.
On the contrary, if we find the ship ahead of the reckoning on one day, and astern of it on another, we have reason to believe that such errors are owing to accidental causes, and not to currents. This seems to have been the case in our passage between England and Teneriffe. But, from the time of our leaving that island, till the 15th of August, being then in the latitude of 12° N., and longitude 24° W., the ship was carried 1° 20ʹ of longitude to the westward of her reckoning. At this station, the currents took a contrary direction, and set to E. S. E., at the rate of twelve or fourteen miles a day, or twenty-four hours, till we arrived into the latitude of 5° N., and longitude of 20° W.; which was our most easterly situation after leaving the Cape de Verde Islands, till we got to the southward. For in this situation the wind came southerly, and we tacked and stretched to the westward; and for two or three days could not find that our reckoning was affected by any current. So that, I judged, we were between the current that generally, if not constantly, sets to the east upon the coast of Guinea, and that which sets to the west towards the coast of Brasil.
This westerly current was not considerable till we got into 2° N., and 25° W. From this station, to 3° S. and 30° W., the ship, in the space of four days, was carried one hundred and fifteen miles in the direction of S. W. by W., beyond her reckoning; an error by far too great to have any other cause but a strong current running in the same direction. Nor did its strength abate here; but its course was, afterward, more westerly, and to the N. of W.; and off Cape Augustine, north, as I have already mentioned. But this northerly current did not exist at twenty or thirty leagues to the southward of that Cape, nor any other, that I could perceive, in the remaining part of the passage. The little difference we afterward found between the reckoning and observations, might very well happen without the assistance of currents; as will appear by the Table of Days’ Works.
In the account of my last voyage[[103]], I remarked, that the currents one meets with in this passage generally balance each other. It happened so then; because we crossed the line about 20° more to the eastward than we did now; so that we were, of consequence, longer under the influence of the easterly current, which made up for the westerly one.
And this, I apprehend, will generally be the case, if you cross the line 10° or 15° to the east of the meridian of St. Jago.
From these remarks I shall draw the following conclusion, that, after passing the Cape de Verde Island, if you do not make above 4° or 5° easting, and cross the line in, or to the westward of, the meridian of St. Jago, you may expect to find your ship 3° or 4° to the westward of her reckoning, by the time you get into the latitude of 10° S. If, on the other hand, you keep well to the east, and cross the line 15° or 20° to the east of St. Jago, you will be then as much to the east of your reckoning; and the more you keep to the eastward, the greater will be your error; as has been experienced by some India ships, whose people have found themselves close upon the coast of Angola, when they thought its distance was above two hundred leagues.