The PETRELS (_Procellaria_, Linn.) are characterised by a gibbous beak, the extremity of which is hooked, and seems made all in one piece, and as if jointed on to the rest of the upper mandible. These birds never dive, and rarely swim; but in their rapid flight they skim over the waves, and actually appear to walk upon the waters. To this habit they owe the name of Petrel, which simply means "Little Peter," in allusion to the miracle of St. Peter, who walked upon the restless waters of the Lake of Genesareth. The family of the Petrels contains several species of very different appearance. They traverse immense distances in their powerful and rapid flight, although they nearly always keep close to the water. They never draw near to the coast except to build their nests, for which purpose they select a little crevice in some steep rock, in which they deposit a large white egg. While sitting upon it, they keep up a low and continual noise, like that of a spinning-wheel. [Illustration: Fig. 112.--The Blue Petrel (_Procellaria cærulea_, Gmelin).] In general, Petrels are not of a very engaging aspect; but they are an invaluable resource to the poor people who inhabit the islands in the frigid seas, who do not object to eat the flesh of these birds, although they principally value them for their warm down and the oil which can be extracted from their stomachs. The quantity of oil which these birds contain is so large that it is used as an article of diet. In the Feroë Islands candles are made from this oleaginous matter. Often, indeed, the islanders make the bird itself serve as a candle to illuminate their gloomy vigils. This is performed by passing a wick through the body of the bird when just killed. These birds appear to love the tempest; they run over the roughest waves, and seem as if they were enjoying themselves as they pass up and down the declivities of the mountains of foam. When the storm is too violent for them, they take refuge upon the nearest rocks, or even on the yards of a passing ship. Sailors, who are confessedly simple and superstitious, take these birds for evil spirits, birds of the devil, harbingers of storms, and so forth, simply because they more frequently see them during a storm. Their black plumage tends to confirm the sailor in his superstition.