230 PHYSICS. 
distance from it towards the desert. The air is generally calm and pure ; 
early in the morning and in the evening, therefore, objects appear in the 
natural positions and distances. But when the ground becomes heated 
by the ascending sun, and with it the lower strata of the air, a tremulous 
motion will be observed in the distance, such as is seen during a hot and 
calm summer day, or such as takes place sometimes over a heated stove. 
Should the air be perfectly calm, so that these inferior strata are not 
disturbed in the least, a direct image of all the villages and other elevated 
objects will be seen in the distance, and under them a reflected image, just 
as if they were situated on the banks of a quiet Jake. The ground thus 
appears to be covered with water, in which the hills stand like islands. On 
approaching nearer, the apparent outline of the water recedes, always 
keeping at a certain distance from the observer. This phenomenon is 
frequently referred to by Arabian authors, as also in the Koran, under the 
name of Serab. It is likewise known in Middle India, where it is called 
Chittram, Dessasur, &c. In many cases, instead of an inverted image 
under the distant objects, only a bright streak of light is perceived, this 
image being too small to be perceptible. Thus we sometimes see the 
ascending sun apparently floating in the air, especially when the eye is 
near the ground. | 
Even at sea, and under quite different circumstances (namely, when the 
ground is colder than the contiguous strata), similar phenomena present 
themselves. In general we may distinguish three different classes of these 
phenomena. To the first belong those cases where distant objects, which 
generally are concealed by the curvature of the earth, become visible for a 
short time. This occurs both by land and sea, especially in the Northern 
Ocean. To the second class belong the cases in which distant objects 
appear surrounded by water, a phenomenon occurring in extended plains, 
and not confined to warm countries, but even found in the Steppes of 
Russia. In the third class are placed those complicated appearances in 
which the object appears double, and sometimes even three-fold; here an 
upward or downward reflection is distinguishable, according as_ the 
secondary inverted image appears above or below the primary. Thus, not 
unfrequently, vessels just visible in the horizon are seen double, a second 
inverted image being observed above an upright one, the mast tops of the 
two pointing towards each other (pl. 24, fig. 20). Again, in the case of a 
vessel whose masts just loom above the horizon, we may see two images, 
one erect, and beneath this another inverted (fig. 21). The latter 
(downward) reflection rarely occurs on land. Professor Vince once saw 
at Ramsgate, on a sultry evening, after a very hot day, the mast of a vessel 
projecting above the horizon, at some distance above it the image of the 
whole ship inverted, and above that again an erect image, the two latter 
separated by a strip of sea. When the vessel approached, the upright 
image vanished first, then the strip of sea, and finally the inverted 
image. 
The time-renowned phenomenon of the Fata Morgana, observed on the 
coast of Calabria, and especially at Reggio, belongs in this place. At a 
404 
