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."

The bridge was the most urgent work. Trees were selected, cut down, stripped of their branches, and cut into beams, joists, and planks. The end of the bridge which rested on the right bank of the Mercy was to be firm, but the other end on the left bank was to be movable, so that it might be raised by means of a counterpoise, as some canal bridges are managed.

This was certainly a considerable work, and though it was skillfully conducted, it took some time, for the Mercy at this place was eighty feet wide. It was therefore necessary to fix piles in the bed of the river so as to sustain the floor of the bridge and establish a pile-driver to act on the tops of these piles, which would thus form two arches and allow the bridge to support heavy loads.

Happily there was no want of tools with which to shape the wood, nor of iron-work to make it firm, nor of the ingenuity of a man who had a marvelous knowledge of the work, nor lastly, the zeal of his companions, who in seven months had necessarily acquired great skill in the use of their tools; and it must be said that not the least skilful was Gideon Spilett, who in dexterity almost equaled the sailor himself. "Who would ever have expected so much from a newspaper man!" thought Pencroft.

The construction of the Mercy bridge lasted three weeks of regular hard work. They even breakfasted on the scene of their labors, and the weather being magnificent, they only returned to Granite House to sleep.

During this period it may be stated that Master Jup grew more accustomed to his new masters, whose movements he always watched with very inquisitive eyes. However, as a precautionary measure, Pencroft did not as yet allow him complete liberty, rightly wishing to wait until the limits of the plateau should be settled by the projected works.