The second operation of making the pieces that go into the structure is simply the following out of the clearly drawn plans furnished by the designing engineers. Different grades of steel and iron are moulded or forged into shape and riveted together, each part being made the exact size and shape required, even the position of the holes through which the bolts or rivets are to go that are to secure it to the neighbouring section being marked on the plan. The foundations for bridges are not always put down by the builders of the bridge proper; that is a work by itself and requires special experience. On the strength and permanency of the foundation depends the life of the bridge. While the foundries and steel mills are making the metal-work the foundations are being laid. If the bridge is to cross a valley, or carry the roadway on the level across a depression, the placing of the foundations is a simple matter of digging or blasting out a big hole and laying courses of masonry; but if a pier is to be built in water, or the land on which the towers are to stand is unstable, then the problem is much more difficult. For bridges like those that connect New York and Brooklyn, the towers of which rest on bed-rock below the river's bottom, caissons are sunk and the massive masonry is built upon them. If you take a glass and sink it in water, bottom up, carefully, so that the air will not escape, it will be noticed that the water enters the glass but a little way: the air prevents the water from filling the glass. The caisson works on the same principle, except that the air in the great boxlike chamber is highly compressed by powerful pumps and keeps the water and river ooze out altogether.