This was done by means of two stakes securely fixed in the sand. Then the settlers, ascending the left bank of the Mercy, soon arrived at the angle formed by the river. There they halted, in order to ascertain if the bridge could be thrown across. The place appeared suitable. In fact, from this spot, to Port Balloon, discovered the day before on the southern coast, there was only a distance of three miles and a half, and from the bridge to the Port, it would be easy to make a good cart-road which would render the communication between Granite House and the south of the island extremely easy. Cyrus Harding now imparted to his companions a scheme for completely isolating Prospect Heights so as to shelter it from the attacks both of quadrupeds and quadrumana. In this way, Granite House, the Chimneys, the poultry-yard, and all the upper part of the plateau which was to be used for cultivation, would be protected against the depredations of animals. Nothing could be easier than to execute this project, and this is how the engineer intended to set to work. The plateau was already defended on three sides by water-courses, either artificial or natural. On the northwest, by the shores of Lake Grant, from the entrance of the passage to the breach made in the banks of the lake for the escape of the water. On the north, from this breach to the sea, by the new water-course which had hollowed out a bed for itself across the plateau and shore, above and below the fall, and it would be enough to dig the bed of this creek a little deeper to make it impracticable for animals, on all the eastern border by the sea itself, from the mouth of the aforesaid creek to the mouth of the Mercy. Lastly, on the south, from the mouth to the turn of the Mercy where the bridge was to be established. The western border of the plateau now remained between the turn of the river and the southern angle of the lake, a distance of about a mile, which was open to all comers. But nothing could be easier than to dig a broad deep ditch, which could be filled from the lake, and the overflow of which would throw itself by a rapid fall into the bed of the Mercy. The level of the lake would, no doubt, be somewhat lowered by this fresh discharge of its waters, but Cyrus Harding had ascertained that the volume of water in the Red Creek was considerable enough to allow of the execution of this project. “So then,” added the engineer, “Prospect Heights will become a regular island, being surrounded with water on all sides, and only communicating with the rest of our domain by the bridge which we are about to throw across the Mercy, the two little bridges already established above and below the fall; and, lastly, two other little bridges which must be constructed, one over the canal which I propose to dig, the other across to the left bank of the Mercy. Now, if these bridges can be raised at will, Prospect Heights will be guarded from any surprise.”