There is a curious, but little known fact, which well illustrates the ravages which the rats can inflict on a hard substance with these little sharp teeth. Many of the elephant's tusks imported into London for the use of the ivory ornament makers, are observed to have their surfaces grooved into small furrows of unequal depths, as though cut out by a very sharp-edged instrument. Surely no man would have taken the trouble to do this, for what would be the profit of his labor? The rats, however, are at the bottom of the secret, or else, clever fellows as they are, they would not have used their chisel-like teeth with such effect. They have found out the tusks which contain the most gelatine or animal glue, a sweet and delicious morsel for the rat's dainty palate; and having gnawed away as much as suited their purpose, have left the rest for the ivory-cutter--he, for his part, is neither unable nor unwilling to profit by the fact marked out by the rat's teeth. The ivory that contains a large amount of gelatine is softer and more elastic than that which does not; and as elasticity is the thing most needful for billiard balls, he chooses this rat-marked ivory, and turns it into the beautiful elastic billiard balls we see on the slate tables in St. James's-street. The elasticity of some of these is so great, that if struck down forcibly on a hard pavement, they will rebound into the hand to the height of three or four feet. Rats have a remarkable instinct for finding out where there is any thing good for food; and it has been often a subject of wonder, how they manage to get on board ships laden with sugar and other attractive cargoes. This mystery has, however, been cleared up, for they have been seen to come off shore to the ship by means of the rope by which she is moored to the quay, although at some distance from the shore. By the same means they will leave the ship when she comes into port, if they find their quarters filling, or filled with water; hence the saying, that "rats always leave a sinking ship" is perfectly true. If, however, the ship be water-tight, they will continue breeding to an enormous extent. M. de St. Pierre informs us, that on the return of the "Valiant" man-of-war from the Havana, in the year 1766, its rats had increased to such a degree, that they destroyed a hundred weight of biscuit daily. The ship was at length smoked between decks in order to suffocate them; and six hampers were for some time filled every day with the rats that had thus been killed.