The following are the names of the places and observations; I omit their geographical situations:

At Badoo,Mungo Park[88]14°6’E. declin.
On the Bâfing,idem 1630
On the Dhioliba,idem 1636
At Yamina,idem 1711
At Sansanding,idem 1740
At Bathurst,Captain Owen[89]1756
At St. Louis,Maritime Annals[90]1732
At St. Mary,M. le Prédour[91]1725
At Cape Roxo,idem 1720

In Mungo Park’s journal of his second expedition, as printed in London, is a tracing of the Gambia,[92] from which a smaller declination would result: it appears evident to me, that some error has crept in here, either in the copying or in the engraving, when the coincidence of the preceding observations from five different travellers is considered, and particularly that of Park’s own observations.[93] To these might be added the declination observed by Major Laing in his first journey, and that which has been observed at Sierra-Leone and other places; but this inquiry would be superfluous. It will be remarked, that the mean term of 16° 55’ declination east, is also the mean between the two observations of M. Caillié; but, as the observation of Badoo seems hither small compared with all the others, I have thought myself entitled to adopt 17° as the mean declination and to subject the whole route to it.

I shall here make another remark respecting the situation of places marked upon the two maps, upon information communicated by others, and not direct observations; it is that the natives are extremely clever at indicating the direction of places at a greater or less distance: they are seldom mistaken in this indication; and they point out with the finger with great accuracy, the direction which must be taken to go in a straight line to a given spot. This observation has been made before. When a certain direction was thus pointed out to M. Caillié, he remarked some particular object on the line, and applied his compass to it at the first favourable opportunity. These bearings have been extremely useful to me for the points situated beyond the route; without this assistance I should have found it impossible to trace, even tolerably, the course of the Dhioliba above Djenné.

COMPUTATION OF THE DAYS’ JOURNEYS.

The geography of countries which have not been explored by observers furnished with instruments is usually reduced to the calculations of days’ journeys. What can be more vague or doubtful than such documents? The most learned discussion (as remarked above), can only elicit feeble scintillations from them. How are contradictory accounts to be reconciled? How are common days’ journeys to be distinguished from double days, or even longer still? It is evident, that itineraries must be examined and compiled from the number of the hours journeys, and not by the days, and there would then be a less degree of uncertainty. Should European travellers themselves compute their lines of march by the days’ journeys? And how happens it that it is not an established rule, in exploring distant and unknown countries, to keep an exact account of the hours and every fraction of time? The journal of M. Caillié, although he has not rigorously complied with this condition, at least presents an uninterrupted continuity of marches measured by time, generally by hours, sometimes by half-hours and even quarters. But for this persevering (and amidst so many fatigues truly laudable) attention, positive geography would have gained very little by these long and toilsome peregrinations.

In a memoir inserted in the eighty-first volume of the Philosophical Transactions, Major Rennell has fixed the day’s journey of a caravan heavily laden at sixteen geographical miles (or minutes of a degree) and one sixth; and that of a light caravan, at seventeen miles and one third. It appears to me, that the second of these results is too small, and especially that an intermediate term should have been established, between the light and the heavy caravans, for there is a vast difference between the two extremes. The former certainly advance more than twenty geographical miles a day, especially, as they are able to continue their journey longer, that is to say, for a greater number of hours each day. Then, again, it would be convenient to fix an intermediate valuation between sixteen and twenty miles, that is to say, the day’s journey of the medium caravans, if they may be so called. I find Major Rennell’s appreciation of the first denomination of days’ journeys confirmed by the experience of the engineers attached to the French expedition, in crossing the deserts near Egypt. We estimated the hour’s march at nineteen hundred toises; eight hours would amount to fifteen miles two hundred toises, or very nearly sixteen geographical miles; the hour’s march would thus be established at two miles, or thirty hours to the degree. In consequence of several calculations much too long to be reported here, I consider the day’s journey of the medium caravan to be eighteen geographical miles and four tenths, the hour being between two miles and two miles and one tenth. I shall not here take the light caravans into account; these travel longer, and the progress of each hour is at the same time greater; but I think it may in many cases be estimated at twenty-two miles, or ten hours of two miles and two tenths. Persons travelling in small parties, without reckoning men mounted on horses or dromedaries, move still quicker.

According to Captain Lyon, the days’ journeys of the caravans are less than twenty English miles, and above seventeen; that is to say, more than the short day, and less than the medium. It is not by the pace either of a pedestrian or of a lightly loaded camel that the progress of a caravan must be measured, but, on the contrary, by that of the man or camel bearing the heaviest burden; for the latter must be waited for by the former, and is perpetually retarding the march; otherwise the usual pace would be much greater than that just fixed. Besides, this reduction of the average value is independent of that which must be allowed for the deviations and turnings which are often unknown: another source of hesitation and of error to geographers.

The earlier journeys of M. Caillié on leaving Kakondy afford an example of the real amount of the day’s journey: the first day, he advanced twenty-three English miles, the following sixteen miles and a half; the third eighteen miles; the average rate nineteen miles and one sixth.[94] The hour’s journey was estimated by him at three miles only, but the construction of the itinerary proves that this computation was more than one tenth too small. The result is twenty one English miles,[95] which is very near eighteen geographical miles and four tenths; all, by my estimation, medium days, (or days of a medium caravan). Indeed, the small caravan to which M. Caillié then belonged may be balanced against these considerations—first, that though small in number it included a woman; secondly, that all were on foot; thirdly, that all were loaded. In traversing the desert, the day’s journeys were of twelve hours and sometimes more;[96] but exhaustion, thirst, and fatigue, prevented both the travellers and camels from keeping up the same pace as on their first departure, and a mile and a half an hour is a large allowance for the march.

I ought here to report the opinion of M. Walckenaer, who has discussed with much sagacity this important geographical question:[97] he fixes at fifteen minutes the amount of a day’s journey of a caravan heavily loaded; this amount falls short by only one sixth of that which I have adopted, from our own experience in Egypt, and which further confirms the opinion of Major Rennell.