"Now all these phenomena, and their various modifications, depend on the different density of the lower strata of the air; and as this difference of density may be occasioned both by heat and moisture, and as heat may be reverberated from the mountain's side as well as from the horizontal surface of the plains, from the sea as from the land--and, further, as contiguous columns of air, as well as horizontal strata, may be of different densities, it is easy to conceive why the mirage may be seen in very different situations, as also why it presents such varied appearances. It will also be evident that any cause which re-establishes the equilibrium of density in the different portions of the air, must cause the illusions of the mirage to vanish."[11] [11] Penny Cyclopædia. It cannot be doubted that those appearances, which sailors call _Cape Fly-away_, the _Enchanted Island_, and the _Flying Dutchman_, etc., are the effects of the mirage. They are the objects of superstition, and thus has an illusive appearance caused many a heart to beat with fear, which never so beat in the tempest or the sea-fight. The eye assists us greatly, indeed almost exclusively, in determining the _motions_ of bodies; yet in this we require experience, and after all we are frequently deceived. When a body moves in a straight line from us, we cannot tell whether it moves or not, and then only come to the conclusion that it recedes in consequence of its becoming more obscure, and from a change in its relative position to other objects which we know to be stationary. When we travel in a railway carriage, the banks, the hedge-rows, and the fields, seem as if gliding with extreme velocity away from us. It is only by a course of reasoning that we know such is not the fact; it is true that the course of reasoning is very short and simple, yet still that reasoning is requisite; indeed, it has been acknowledged only in modern times that the sun is stationary, and that the world moves. Bodies moving with extreme velocity produce a continuous impression on the eye; thus, for example, if a stick burning at one end be rapidly whirled, the burning end produces the appearance of a fiery circle. The spokes of a coach-wheel, rolling round very rapidly, present the appearance of an indistinct expansion. On the contrary, bodies moving with extreme slowness appear to casual observation as stationary; thus the act of growing, in ordinary plants and animals, is not appreciated, though the result is demonstrated in due time. The eye, moreover, may be deceived by artificial representations, as by painting (take the Diorama as an example) or by models. We have seen flowers so truthfully modelled in wax that, by the eye alone, no distinction between them and natural flowers could have been detected.