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 heaving, 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 a-head of the reckoning on one day, and a-stern 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° 20' 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 toward the coast of Brazil. 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 115 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 N. 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 Day's Works.[90]
[Footnote 90: The curious reader will find some interesting, though not decisive, remarks concerning the currents of the Atlantic Ocean in Clerke's Prog. of Mar. Disc. vol. i. p. 358.--E.]
In the accounts of my last voyage, I remarked, that the currents one meets with in his 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 E. of the meridian of St Jago.
From these remarks I shall draw the following conclusion, that after passing the Cape de Verde Islands, 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 E. and cross the Line 15° or 20° to the E. of St Jago, you will be then as much to the E. 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 200 leagues.
During the whole of our passage from England, no opportunity was omitted of observing, with all the attention and accuracy that circumstances would permit, the variation of the compass, which I have inserted in a table, with the latitude and longitude of the ship at the time of observation. As the longitude may be depended upon, to a quarter or half a degree at most, this table will be of use to those navigators who correct their reckoning by the variation. It will also enable Mr Dun to correct his new Variation Chart, a thing very much wanted.
It seems strange to me, that the advocates for the variation should not agree amongst themselves. We find one[91] of them telling us, as I have already observed, "that with 8° W. variation, or any thing above that, you may venture to sail by the Cape de Verde Islands by night or day, being well assured, with that variation, that you are to the eastward of them." Another, in his chart,[92] lays down this variation ninety leagues to the westward of them. Such a disagreement as this, is a strong proof of the uncertainty of both. However, I have no doubt the former found here, as well as in other places, the variation he mentions. But he should have considered, that at sea, nay even on land, the results of the most accurate observations will not always be the same. Different compasses will give different variations; and even the same compass will differ from itself two degrees, without our being able to discover, much less to remove, the cause.
[Footnote 91: Nichelson.]
[Footnote 92: Mr Dun.]
Whoever imagines he can find the variation within a degree, will very often see himself much deceived. For, besides the imperfection which may be in the construction of the instrument, or in the power of the needle, it is certain that the motion of the ship, or attraction of the iron-work, or some other cause not yet discovered, will frequently occasion far greater errors than this. That the variation may be found, with a share of accuracy more than sufficient to determine the ship's course, is allowed; but that it can be found so exactly as to fix the longitude within a degree, or sixty miles, I absolutely deny.[93]