Cobras are very fond of eggs, and if they can find a rat-hole which opens into a hen-house they will often take advantage of it in order to rob the nests. But sometimes, when they have swallowed several eggs, and the hole happens to be a small one, they cannot crawl out again, and are found and killed when the house is opened in the morning. THE PUFF-ADDER Quite as deadly is the puff-adder, of Africa, which has a way of lying almost buried in the sand, so that it is not easily seen; and if it is disturbed it does not crawl away, as most poisonous snakes will do, but remains quite still, merely drawing back its head in order to strike. When fully grown it is about six feet long, and its poison is so deadly that even a horse has been known to die within two or three hours of being bitten. This snake is called the puff-adder because it draws in a very deep breath when it is annoyed or irritated, and puffs out its whole body to nearly double its proper size. It then allows the air to escape gradually, with a kind of sighing noise, draws in another deep breath, and so on over and over again. PIT-VIPERS Australia, also, has some snakes whose bite is very deadly; and in general the tropics abound in these dangerous reptiles. This is as true of America as elsewhere, but all the American venomous serpents are of a kind peculiar to this continent, called pit-vipers. Some of them have rattles at the end of the tail and some lack this appendage, but all are much alike. Certain of the most dreaded, such as the fer-de-lance and the bushmaster, belong to the West Indies and Northern South America; but really the worst of the whole bad lot, because of its great size and sullen ferocity, is the huge diamondback rattlesnake of the Southern States. It is in some cases longer and heavier than any other known venomous snake; and its bite, if the wound is well poisoned, means almost immediate paralysis and death. RATTLESNAKES Several different species of rattlesnakes are scattered over the United States, and in some places, as on the hot dry plains of the Southwest, and in the arid mountains of Utah and California, are numerous enough to be troublesome. The cutting away of forests, draining of swamps, and cultivation of prairies, soon destroy these pests in thickly settled regions; but where rocky hills occur they linger for a long time, because the breaks and little caves among the ledges offer them secure retreats, winter homes where they sleep in safety, and proper nurseries for the young, which are not produced from eggs, as in the coluber family, but are born alive.