UH I li li A P P I N D I X . xxvii direction* as well as the particular nature, i and tendency, o f the curves, o f the lines o f equal quantities ? . .. . 1 ; T h e v aria tion lines on the .globe have o ccupied a good deal o f m y attention a t différent periods o f: m y life,, and .therefore, the application o f such new observations as the assiduity ;and kindness o f m y friends; had; proeu®d, fo r me on this occasion, was less difficulty than i f the subject had been new. to m e . A dissertation on the subject,..wiould be b u t o f place, h e re ; and; therefore I shall o n ly g iv e the rë s iilt.o f my.inquiries, in abstract,; after p re mising, that the theoretical part; beldpging to, the interior o f A f r ic a , is . founded, on a supposed continuation o f those lines o f equal quantities,.whose tendency has. been already ascertained, in thè surrounding,seas^ I, am perfe c tly awâre, that some may re g a rd the assumption as to o great : h u t they, will no doubt admit, at the same;timeyithat.it,is;difficult to con c e iv e a more, prob able arrangement;; and what is much more to die, purpose, is, that i f we are compelled to abandon thè System, in the gross, the quantity o f v a r iation in the line o f M r . P a rk ’ s travels, cannot b e g tead y différent from what we have assumedh 'F o r , whether thé line o f 18° in the south A tla n t ic , h e a continuation o f that in the north A tlan tic , o r o f that in the Indian sea, much the same result will fo llow : on ly that in the fo rmer case, the quantity; will b e somewhat greater. I t wQuld appear, that between the E a s t Indies and South Am e r ic a , E u ro p e and South A fr ic a , there are fo u r distinct sets o f what may b e termed concentric curve$ of variation lines, On theiglobq,; and whose highest points o f co n v e x ity are opposed to each other, within the great body o f No rthern A f r ic a . T h e accompanying sketch will best explain it.* I t w ould appear moreov er, that from the p lac e o f opposition o f these cu rv e s , in Africa, where * This sketch is not pretended to be minutely accurate ; ilnbeing morally impossible to procure recent observations in every, part, from the rapid .change that takes place in- the quantity o f the variation, in one and the same spot. However,, the observations that determine the course o f the lines in the Atlantic (and which are marked on the sketch) are from observations* so late as 1793. Thè’ same is to be said o f those in the western quarter o f the Mediterranean ; and those beyond the Cape of Good Hope, to longitude 30® east, are o f the year 1789. ■ It is obvious that a critical knowledge o f the quantity of the variation in any particular d 2
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