It often happens that on their march they come to water, and, as they always advance with total disregard of difficulties, they must needs invent some very ingenious way of overcoming the difficulty. One of them climbs a branch which overhangs the water, clasps the undermost twig very tightly, and allows itself to hang from it. Another at once follows, and suspends itself from its comrade in like manner, the powerful and sicklelike jaws doing their duty as well as the legs. A chain of Ants is thus speedily formed. When the lowermost Ant touches the water, it merely spreads all its legs, and awaits the development of events. Another runs over it, holds to the first Ant by its hind-legs, and stands in the water, spreading its limbs as much as possible over the surface. Ant after Ant descends, until quite a long chain of the insects is formed, and is swept downwards with the stream. By slow degrees the chain is lengthened, until the Ants at its head are able to seize the bank on-the opposite side of the water. When they have succeeded in doing so, the bridge is complete, and over that living bridge will pour a whole army of Driver-ants. Even in those cases where this mode of travelling would be too perilous on account of the rapid torrent, the Ants contrive to suspend themselves in long strings until they effect a communication with the trees of the opposite bank. It is, perhaps, needless to give more than a passing reference to the Suspension-bridges made by Spiders, by means of which they can traverse considerable distances. The similar bridge of the Little Ermine Caterpillar has already been mentioned, when treating of the subject of Double Walls. ARCHITECTURE CHAPTER IV. LIGHTHOUSES.--THE DOVETAIL.--THE DAM.--SUBTERRANEAN DWELLINGS.--THE PYRAMID.--MORTAR, PAINT, AND VARNISH.