METEOROLOGY. 229 
Rays proceeding from the sun and other luminous bodies are refracted 
on their entrance into the atmosphere. As this, however, is composed of 
innumerable strata, increasing in density to the surface of the earth, these 
rays must be continually refracted, and thus move in curved paths which 
are concave to the earth. The consequence of this is, that unless a 
heavenly body is actually in the zenith, where no refraction can take place, 
its place, as seen by our eyes, will be higher than its true place. The 
amount of this displacement increases from the zenith to the horizon. 
Near the horizon it is half a degree, at an elevation of 45° it is one 
minute, and at 75° only one fourth of a minute. The sun (as also other 
luminaries) is thus seen in the morning before his actual appearance above 
the horizon, and in the evening after he has already set. The day, then, 
is lengthened by refraction, this lengthening amounting during the longest 
days to eight or nine minutes in Germany, and at higher latitudes to much 
more—to days and weeks ir. the polar regions. The same cause produces 
the oval form of the sun when just above the horizon, his lower limb being 
more refracted than the upper, his vertical diameter being thus abbreviated 
while the horizontal suffers no change; the result is the apparent elongation 
of his disk. A similar effect is produced on terrestrial objects, for which 
reason we distinguish a terrestrial from an astronomical refraction. | 
Refraction varies at different times, owing to its dependence on a 
variable condition of the atmosphere. For this reason the horizon does not 
appear always in the same place; it seems lower when the air at the 
surface of the ground is warmer and rarer than at an elevation of some 
feet; and higher when the groundis colder. Should the ground be strongly | 
heated we shall observe a lively shaking or tremulous motion of objects, 
owing to the existence and combination of air strata of different densities 
and unequal refractive powers. 
When the sun is very hot and the air calm, the lower strata of air, heated 
by the sun, often have a less density than those above them, without 
changing their position. In pl. 24, fig. 19, let ab be the horizontal surface 
of the earth, 2 an elevated point or object; let the eye of an observer be at 
p. He will see first a direct image of the object, 4, in the direction ph, in 
which the rays, being only slightly deflected from a right line, will only 
produce irregularity in the outline of the image. Other rays, however, 
coming from h, follow the path hilmnp, since the ray, hi, which traverses 
strata c, c', c’’, c/’, of decreasing density, is continually refracted from the 
perpendicular, becoming more and more acute-angled to the horizon. After 
traversing a sufficient number of these strata it ceases to be refracted any 
longer, and then it is reflected, reaching the eye in the direction map. The 
observer will thus see an inverted image of the object, 4, in the direction 
pt. 
Phenomena of this kind generally occur in hot countries, especially in 
deserts, as, for instance, in Egypt, where the French army was frequently 
bitterly deluded by this mirage. In Lower Egypt the ground forms a vast 
and perfectly horizontal piain, exposed to the overflew of the Nile. The 
villages are there built on sma!l hills on the bank of the river, or at some 
403 
