348 
TRAVELS IN NORTHERN AFRICA. 
CHAP. IX. 
always appearing very distant when the sun is on them; and it has 
often happened, that I have been startled by seeing a man or camel 
rise close to me, on the top of one of the apparently distant hills. 
The excessive dryness of the Desert is in some places very extra- 
ordinary, particularly to the southward of the Soudah mountains, 
where, in going to as well as coming from Fezzan, I observed that 
our clothes, and the tails of our horses, emitted electric sparks. 
Water is not to be found by digging in all parts of the Desert ; 
but is more particularly difficult to find in the Sereer, or gravel, 
which generally lies over sand stone. In tw^o instances I have seen 
remains of pits which had been dug to one hundred feet v/ithout 
coming to water. The wells which are on the Desert are generally 
found in Wadeys or in the sandy country ; and in all those I have 
seen, the water was salt and putrid, but the putrescence diminished 
after a quantity had been drawn. Some wells have only a sufficiency 
for the supply of five or six horses at once, and are a long time before 
they again fill. These wells which were so scantily supplied, I ob- 
served, w ere always in a soft clayey rock ; but those which kept 
themselves tolerably full, were in a yellow clay. The depths vary 
from 6 or 8 feet to 70 or 80. 
In almost every part of the stony desert, small piles of stones 
are frequently discovered, which are erected by travellers as marks 
to direct them across the country, or in the event of their missing 
their route, to assist them again to find it. These little heaps are 
called Aalum or "teachers;" and some become so remarkable, 
as to acquire other names, and to be favourite resting-places for 
Kaffles. 
About the beginning of April, a dangerous fever broke out and 
was making great ravages in Tripoli, many of the inhabitants dying 
