Letter from Dr Oudney to Professor Jameson. 881 
has shrunk so that they are rendered useless. The tube of one 
attached thermometer is bent like a bow, from being confined ; 
in another, the pressure of the glass has broken the brass-clasps. 
The hygrometer of Kater generally stands at 88, or 88.5. But 
instruments affected by the hygrometrical state of the air, and 
by sand, are useless in climates such as this. I mention sand, 
for instruments that act by delicate wheels are very soon de- 
ranged, and disappoint the scientific inquirer. The barometer 
varies a little ; there is a change particularly from 11a.m. to 
8 or 9 P- m. ; the mercury becomes depressed in general about 
the 20th part of an inch, sometimes a 10th. During northerly 
winds the mercury rises, and, in general, the stronger the breeze 
the greater the rise. From that I have been able to predict 
strong breezes from that direction. On the contrary, southerly 
winds cause depression, and that commonly in proportion to the 
violence of the wind. I have not yet calculated the mean 
height, but it must be about 28.500 inches at the temperature 
of 80° Fahr. Water boils at 207 Fahr. both which give a consi- 
derable elevation. I have anxiously searched for springs, but 
have found none fit for an accurate result. The whole of the 
country is a spring, if I may use the expression, for water 
comes bubbling up on digging a few feet. Its temperature, then, 
is affected by the earth, and the heat indicated of no use in de- 
termining the mean temperature of the place, and consequently 
its elevation. The abundance of water in a country where it never 
rains, and where no dew falls, is curious and interesting to the 
philosophic inquirer. It is not generated in the earth, and it 
cannot be supplied by the sea. From whence then does it 
come? Is it from the tropical rains? Or rivers lost in the 
earth ? The supply is constant, and the wells yield as much at 
one time as at another. The supposition that appears most pro- 
bable to me is, that the countries to the southward are much 
higher than this ; that the rain during the rainy season pene- 
trates a considerable way into the soil, till it meets strata resist- 
ing its descent ; and that it then flows along them like a river, to 
countries far distant. My explanation may be confused, but, on 
reflection, I think it the most probable. There is another in- 
teresting feature in Fezzan, — the continual formation of salt on 
the surface of the sand. In travelling along, the different sta- 
