As we resume our way along an undulating road, we pass through a district which, despite its northerly exposure, seems able to support a large population, and numerous tidy cottages cluster here and there along the roadside. By-and-by the cultivated strip becomes narrower, the sandy beach of Aikerness gives place to the rocky shores of Burgar, and the road turns inland with a steep incline to dip down on the other side of the ridge towards the Loch of Swannay. Here we shall find it well worth our while to make a somewhat longer halt than before, and, leaving our bicycles, we turn to climb Costa Hill, and to view the wild cliffs at Costa Head. From the hill we look down upon the mysterious green islet of Eynhallow, the “Holy Island,” where the ruins of an ancient monastery have been traced, and round which more than the usual crop of legends has sprung up. A fair contrast it offers to the bold, rocky cliffs of Rousay just beyond. If it happens to be the time of spring tides, and the ebb is running out, we shall see at this place one of the most impressive sights which our coasts present. However calm be the sea, as soon as the tide begins to gather strength, the channels on either side of Eynhallow for some distance out to sea become a mass of heaving, foaming billows, reminding one of the long stretch of boiling rapids below the Falls of Niagara. And that is just what this “roost” is—rapids on the course of the tidal river which is now sweeping westward through Eynhallow Sound. When we look at our pocket map, we see that on each side of the islet the depth of water is only about five fathoms. In about a quarter of a mile it becomes ten fathoms, and within a mile of the west end of the island twenty fathoms. Thus the tidal river first passes over a ridge on each side of Eynhallow, where it is less than thirty feet deep, and then plunges down a slope which dips nearly one hundred feet in a mile. If there is a long swell rolling in from the Atlantic, as there often is on our western shores, the turmoil is increased, and the boiling fury of Burgar Roost, as it is called, is a sight which it is worth going far to see. The roost which is formed in Hoy Sound with a strong ebb-tide is due to similar causes but there the dip in the sea-bottom is not so steep. When the tide turns, the change seems almost magical, and in a short time there may be not a ripple on the water to mark the scene of this mad dance of the billows. [Illustration: _Birsay, the Barony._] The cliffs at Costa Head are the highest on the Mainland, but we can only see them from above, and thus we lose much of their wild grandeur. We enjoy, however, an impressive view of the cave-pierced shores of Rousay, and of the stern ramparts of Noup Head, in Westray, with its sentinel lighthouse.