347. =Mirages.=--Sometimes inequalities occur in the density of the lower portions of the atmosphere, causing, of course, unequal refraction, and producing some strange appearances, termed _mirages_. For example, at Ramsgate, on the coast of England, there was seen, at one time, as represented in Fig. 235 (p. 268), a ship at such a distance that only her topsails were visible; and above in the air there were two complete images of the ship, the uppermost being erect and the under one inverted. Captain Scoresby, in a voyage to Greenland, saw an inverted image of a ship so well defined that he decided that it was the image of his father's ship, the _Fame_, which was afterward verified. The ship itself was at that time at a distance of 30 miles. An incident in the early history of the author's place of residence may be cited as an example of mirage. A ship left for England freighted with a valuable cargo, and having on board a large number of the best citizens of the colony. Some time after there was immense excitement in New Haven, because the inhabitants saw, with great distinctness, what they supposed to be this vessel, at only a little distance, apparently sailing against the wind. But it soon disappeared from view, part after part, until the whole was gone. The ship itself was never heard from, and it was supposed at the time that this appearance was a manifestation of Providence for the purpose of informing the colonists what had become of their friends. But what was seen was undoubtedly the reflected image of this or some other ship. It is such appearances as these that have given rise to the stories which have been sometimes told of phantom ships. Mirages are very common in the extensive deserts in hot climates, exhibiting to the eye of the traveler various deceptive appearances, as islands, lakes, etc. In Bonaparte's campaign in Egypt such an appearance caused whole battalions of thirsty soldiers to rush forward, supposing at the moment that a plentiful supply of water was at hand. [Illustration: Fig. 235.]