2l8 
GEOGRAPHICAL 
a compound winding arifes : and I have found by long experience, 
that one mile in eight mnft be dedu(S^ed, to reduce the road 
meafure, on fuch a length, to horizontal meafure. When a line 
of difknce is extended to 500 miles and upwards, the rule be- 
comes much more vague than when applied to moderate dif- 
tances ; becaufe it often happens (and more particularly in un- 
improved countries) that obfiacles prefent themfelves, and give 
an entire new dire6:ion to the courfe of the road; although the 
two parts of it, confidered feparately, may have only an ordinary 
degree of crookednefs. However, as fome of the lines of dif- 
tance applied to the prefent fubjeft, are from thirty to forty days 
journey, it becomes neceffary that fome general rules fhould be 
^adopted. It happens that examples are furnifhed, in two cafes, 
■on very longjournies, where the real dillances between the ter- 
minating points of the routes are nearly known: fuch is that of 
fifty-three days journey, betv/een the Capital of Fezzan and 
Cairo ; and the mean horizontal diftance for each day, is four- 
teen and a half geographic miles, or thofe of lixty to a degree. 
■I confefs I fhould have expeded much lefs. The other example 
is between Arguin and Gallam : there forty days produce thir- 
teen miles for each day; and this is conformable to my expec- 
-tations. In the exam.ples of fmall diftances, fuch as fix days 
journey, fixteen miles per day is the refult, and is confillent. 
For 
