I have anxiously searched for springs, but have found none fit for an accurate result. The whole 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 is of no use in ascertaining the mean temperature of the place, and consequently its elevation. The abundance of water in a country in which rain scarcely ever falls, and in which there is no dew, is a curious and interesting circumstance 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 is it from rivers lost in the earth? The supply is constant, and the wells yield as much at one time as another. The supposition that appears most probable to me is, that the countries to the southward are much higher than this, that during the rainy season water penetrates a considerable way into the earth, till it meets with _strata_ resisting its farther descent, and then that it flows along these like a river to far distant countries. My explanation may be censured, but still on reflection, I regard it as the most probable that I can think of. There is another interesting feature in Fezzan, namely, the constant formation of salt on the surface of the sand. In travelling along, the different stages of the process are very distinctly observable. First, a thin crust is formed like hoar frost, and this continues to increase in thickness. In some places the layer of salt is a foot deep. At present I must defer the explanation of this phenomenon till I shall have time to enter upon the description of the geological structure of Fezzan—a thing I hope I shall soon be enabled to do. In the mean time the courier is waiting, and the camels are ready to convey us to Ghaat. My worthy friend Clapperton and I are just on the point of setting out on a visit to that country, that we may render our delay here as serviceable as possible. The Tuarick country is, perhaps, interesting only on account of its inhabitants, who are a brave, independent race. Clapperton and I have been at Tucta, and we were both struck with the general uniformity of the scenery and the structure. He desires to be kindly remembered to you, and intends to write you soon. We expect to be able to set out for Bornou in two or three months.”