der. "It is difficult to believe that the swamp and lagoon on the
starboard were once covered with fertile fields, watered by two of the
branches of the Nile, where wheat was raised in abundance, from which
Rome and other countries were supplied with food."

"What vast flocks of birds!" exclaimed Mrs. Woolridge.

"Those are flamingoes, just rising from their resting-place," added the
captain. "They were white just now as we looked at them; notice the
color of the inside of their wings, which are of a rose-tinted pink."

"But what became of the wheat-fields that were here?" asked Mrs.
Blossom, after they had observed the wild birds for a time.

"The sea broke in and covered the rich lands with sand and salt; and
there are towns buried there now."

"Goodness, gracious!" almost screamed Mrs. Blossom. "There's another
steamer sailing on the land!"

"It appears to be so, but is not so," replied the commander.

"It is really so," added Mrs. Woolridge; and all the party gazed with
interest at the phenomenon.

"Only apparently so," the captain insisted.

"Please to explain it to us, Commodore," said Miss Blanche, who had long
ago applied this title to him.

"With pleasure, Miss Woolridge. It is the mirage, from the Latin
_miror_, to wonder, which appears to be what you are doing just now. The
steamer you see sailing along the shore is an optical illusion, a
reflection, and not a reality. Refraction, which is the bending of the
rays of light, produces this effect. If you look at a straight stick set
up in the water, it will appear to be bent, and this is caused by
refraction. The learned gentlemen present will excuse me for going back
to the primer of physics."

"We are quite satisfied to have the memory refreshed," replied the
doctor.

"The air around us is of different densities, which causes the rays of
reflection of our ship to be bent, sending the image up on the shore.
What sailors call 'looming,' often seen on our own shores, is produced
in the same way; and we often see an island, or a vessel, looming up
away above the water, from which it is sometimes separated by a strip of
sky. The mirage is often seen in the desert, with a whole caravan up in
the air, sometimes upside down.

"An object is often seen when at a considerable distance from it. In the
Arctic regions ships below the horizon, or hull down as sailors phrase
it, are revealed to other ships far distant by their images in the air.
From Hastings,