He quickens his pace, enjoying in anticipation the pleasures of a refreshing draught; but, as he approaches, it recedes or entirely disappears; and standing upon its apparent site, he is ready to doubt his own vision, when he finds but a parched sand under his feet.

It is not until he has been thus a dozen times deceived, that he is willing to relinquish the pursuit, and then, perhaps, when he really does see a pond, he will pass it unexamined, from fear of another disappointment.
The philosophy of these false ponds I have never seen satisfactorily explained.

They have usually been attributed to a refraction, by which a section of the bordering sky is thrown below the horizon; but I am convinced that they are the effect of reflection.

It seems that a gas (emanating probably from the heated earth and its vegetable matter) floats upon the elevated flats, and is of sufficient density, when viewed obliquely, to reflect the objects beyond it; thus the opposing sky being reflected in the pond of gas, gives the appearance of water.
As a proof that it is the effect of reflection, I have often observed the distant knolls and trees which were situated near the horizon beyond the mirage, distinctly inverted in the "pond." Now, were the mirage the result of refraction, these would appear on it erect, only cast below the surface.

Many are the singular atmospheric phenomena observable upon the plains, and they would afford a field of interesting researches for the curious natural philosopher.
We had a pleasant journey, although sometimes pressed pretty hard by hunger.