Birds, as "denizens of the air," are the surest indicators of weather changes. Thus, when swallows fly high, fine weather is to be expected or continued; but when they fly low, or close to the ground, rain is almost surely approaching; for swallows follow the flies and gnats, which delight in warm strata of air. Now, as warm air is lighter, and usually moister than cold air, when the warm strata of air are high there is less chance of moisture being thrown down from them by their mixture with cold air; but when the warm and moist air is close to the surface, it is almost certain that, as the cold air flows down into it, a deposition of water will take place. When Seagulls assemble on the land, very stormy and rainy weather is approaching. The cause of this migration to the land is the security of these birds finding food; and they may be observed at this time feeding greedily on the earth-worms and larvæ driven out of the ground by severe floods; whilst the fish on which they prey in fine weather in the sea, leave the surface, and go deeper in storms. The search after food is the principal cause why animals change their places. The different tribes of the wading birds always migrate when rain is about to take place. There is a bird which takes its name from its apparent agency in tempests. Such is the Stormy Petrel, which name Hawkesworth, in his "Voyages," mentions the sailors give to the bird, but explains no further. Navigators meet with the Little Petrel, or Storm Finch, in every part of the ocean, diving, running on foot, or skimming over the highest waves. It seems to foresee the coming storm long ere the seamen can discover any signs of its approach. The Petrels make this known by congregating together under the wake of the vessel, as if to shelter themselves, and they thus warn the mariner of the coming danger. At night they set up a piercing cry. This usefulness of the bird to the sailor is the obvious cause of the latter having such an objection to their being killed. Mr. Knapp, the naturalist, thus pictures gulls, describing the Petrel's action:--"They seem to repose in a common breeze, but upon the approach or during the continuation of a gale, they surround the ship, and catch up the small animals which the agitated ocean brings near the surface, or any food that may be dropped from the vessel. Whisking like an arrow through the deep valleys of the abyss, and darting away over the foaming crest of some mountain-wave, they attend the labouring barque in all her perilous course. When the storm subsides they retire to rest, and are seen no more."