APPENDIX. XXvii 
direction, as well as the particular nature, and tendency, of the curves^ of 
the lines of equal quantities? 
The variation lines on the globe have occupied a good deal of my atten- 
tion at different periods of my life, and therefore the application of such 
new observations as the assiduity and kindness of my friends had procured 
for me on this occasion, was less difficult, than if the subject had been new 
to me. A dissertation on the subject, would be out of place here; and 
therefore I shall only give the result of my inquiries, in abstract; after pre- 
mising, that the theoretical part belonging to the interior of Africa, is 
founded on a supposed continuation of those lines of equal quantities, whose 
tendency has been already ascertained, in the surrounding seas. I am per- 
fectly aware, that some may regard the assumption as too great: but they 
will no doubt admit, at the same time, that it is difficult to conceive a more 
probable arrangement : and what is much more to the purpose, is, that if 
we are compelled to abandon the system, in the gross, the quantity of vari- 
ation in the line of Mr. Park's travels, cannot be greatly different from what 
we have assumed. For, whether the line of 18 0 in the south Atlantic, be 
a continuation of that in the north Atlantic, or of that in the Indian sea, 
much the same result will follow: only that in the former case, the quantity 
will be somewhat greater. 
It would appear, that between the East Indies and South America, Europe 
and South Africa, there are four distinct sets of what may be termed con- 
centric curves of variation lines, on the globe, and whose highest points of 
convexity are opposed to each other, within the great body of Northern 
Africa. The accompanying sketch will best explain it.* It would appear 
moreover, that from the place of opposition of these curves, in Africa, where 
* This sketch is not pretended to be minutely accurate ; it being morally impossible 
to procure recent observations in every part, from the rapid change that takes place in 
the quantity of the variation, in one and the same spot. However, the observations that 
determine the course of the lines in the Atlantic (and which are marked on the sketch) 
are from observations so late as 1793. "The same is to be said of those in the western 
quarter of the Mediterranean ; and those beyond the Cape of Good Hope, to longitude 
30 0 east, are of the year 1789. 
It is obvious that a critical knowledge of the quantity of the variation in any particular 
d 2 
