cultivated life–to all of which you would nevertheless prefer a refreshing draught of cold water–there emerges in the plain before your astonished eyes a beautiful lake. Its surface looks like crystal; the wind moves but slightly the wide sheet of water; but the faster you hurry forward, the nearer you approach it, the sooner you will be disenchanted; the lake disappears again before your presence; and when you arrive at the very spot, you perceive nothing but the same hard, dry, parched soil, over which you have travelled all day. This is the celebrated "mirage," (false ponds; fata morgana.) Though it also appears in other parts of the prairie, it is nowhere so common, so deceptive, and so well developed, as here. In examining the causes which produce it at this high plain, I have arrived at the following conclusions:
The phenomenon of mirage requires–
- A wide high plain, with extensive horizon, and but slight undulations of the ground.
- A dry, hard ground, either quite barren, or but coated with parched and isolated vegetation, like the short buffalo grass.
- Dry and warm weather, with a clear sky. On such days, and less in the morning and evening, but rather when the sun has the most power, mirage is the most frequent and the plainest.
- A slight hollow in the undulating plain, however insignificant it may be, producing a background. Where this low background is interrupted by the horizon, on that place the mirage grows more dim and disappears entirely.
- The distance of several miles from the stand of the observer. The nearer one approaches, the more indistinct becomes the mirage, and it changes at last into a glimmering of the air, such as can be seen on hot summer days upon dry, solid, macadamized roads, from which the rays of the sun are powerfully reflected.
- The mirage is therefore the effect of a strong reflection of the rays of the sun from the ground, seen out of a certain distance, on certain localities.
- That objects, being near the mirage, as trees, animals, men, &c., are seen double, can also be explained by the following law of reflection:
When two strata of air, one of common middle temperature and density, and the other hotter, meet together, an observer, standing also in a common temperature and looking at an object near where the two strata meet, will see that object double, directly in the stratum of common air, in which he stands himself, and indirectly by reflected light in the hotter stratum. The direct image will stand upright; the reflected one inverted.
But let us return to our caravan. While we were travelling to-day over the lonesome plain, men and animals quite tired and exhausted, on the rising of a hill before us quite suddenly appeared a number of savage looking riders on horseback, which at first sight we took for Indians; but their covered heads convinced us soon of our mistake, because Indians never wear hats of any kind: it was a band of Ciboleros, or Mexican buffalo hunters, dressed in leather or blankets, armed with bows and arrows and a lance–sometimes, too, with a gun–and leading along a large train of jaded pack animals. Those Ciboleros are generally poor Mexicans from the frontier settlements of New Mexico, and by their yearly expeditions into the buffalo regions they provide themselves with dried buffalo meat for their own support and for sale. Their principal weapon is the lance,
