which rapidly grew up. In place of a separate reef for each island, one larger reef was thus made, which enclosed all the islands. There are about a dozen islands in all, of different sizes. The largest is about sixty miles in length by about fifteen in breadth, and the remainder range from twenty miles to half a mile in diameter. By this means many hundreds of square miles have been gradually added to the space suitable for human habitation. These new spaces were all amply supplied with air-tubes and electric lamps. The old barriers that had separately surrounded each island were removed, and the additional space was rapidly covered with submarine houses and villas, thus providing accommodation for the ever increasing population. The new reef quickly rose above the level of the highest tides, opposing a perfect barrier against the inroads of the sea. But as it was requisite to secure a continuous flow of fresh water through the enclosed lake, the reef was perforated with tunnels in every direction, which, by means of a system of self-acting valvular doors, permitted the influx and efflux of the ocean currents in a gentle and equable manner. In order to maintain the water of the inland sea at a uniform temperature, these tunnels were pierced at different levels. The water of the ocean near the surface is of a much higher temperature than the deeper water; and when the enclosed water becomes too warm the fresh water is admitted chiefly by the lower tunnels; when it is too cool the influx through the more superficial tunnels soon raises it to the required temperature. By self-acting machinery the upper valves open as the temperature lowers, the lower as the temperature rises. Special inspectors and engineers are appointed to maintain the tunnels
