To enable the subaqueous portions of the piers to be built, coffer-dams were erected around the sites, the space within being emptied and kept clear of water by means of powerful pumps. By this means the workmen were enabled to carry out their task of securing the fabric to the solid rock on the dry coral sea-bed. Where the water ran up to a depth of 30 feet, and the situation was exposed to the full fury of gales and of the Atlantic, caissons were sunk for the purpose of constructing the piers to above water-level, the men working in compressed air. The material for constructional purposes was prepared on large, well-equipped floating plants anchored near by. The timber moulds to form the shape of the arches were fashioned and bolted together on dry land, and towed out to sea by tugs to the point of erection and there set in position. Some of these series of arches on the amphibious section of the railway are only a few hundred feet in length; others measure as many thousands of feet from end to end. For instance, between Long and Grassy Keys--the islands are known as “keys”--the over-sea viaduct is 2 miles from end to end. The viaduct work was confined to the deepest parts of each channel, being approached from either end over a substantial earthen embankment. Some idea of how this expedient saved the costly task of bridge-building may be obtained from the fact that whereas the distance by the line between Grassy and Long Keys is 29,544 feet--5.6 miles--the approach embankments aggregate 19,100 feet of this total, the long, symmetrical line of arches totalling 10,444 feet. In the case of the gap between two other keys the water is closed by an embankment 21,800 feet in length. In another instance the earthen structure stretches for 11,950 feet to connect Upper and Lower Matecumbe, but inasmuch as this channel is used by vessels, the navigable channel is spanned by a drawbridge 120 feet in length to permit vessels to pass between the Atlantic and the Gulf of Mexico. In the first 78 miles of track running out to sea from the mainland no less than 14 miles represent bridge-work, the remaining 64 miles being carried out on embankments across the islands and shallow straits, or by timber trestling.