So far as the first condition is concerned, the proper height of a lighthouse-tower is easily ascertained, when the distance is determined at which its rays should be visible. This distance will necessarily depend on the character of the neighbouring seas, and the nature of the reef, rock, or shallow from which the lighthouse is to warn the navigator; but, once determined, the elevation of the tower will easily be calculated by means of the known relations existing between the form of the earth, the effects of atmospheric refraction, and the proper height of an object which is to be seen from a given distance. The state of the atmosphere at any particular point is also an important point of consideration. It is quite possible that the lighthouse, when erected at what seems a suitable elevation, may be rendered useless by a prevalence at that elevation of dense mists and heavy fogs. In 1785, the Trinity Board commenced the erection of a lighthouse on the summit of St. Catherine’s Down, in the Isle of Wight, and from so lofty an altitude it might well be supposed that its radiance would illuminate the Channel for leagues around. But, unfortunately, the crest of St. Catherine’s is, for the greater part of the year, enveloped in cloud and mist, which effectually prevents the escape of a single ray of light; and, consequently, the Trinity Board were compelled to abandon their design. The shell of the building still crowns the bleak summit of the down, as a warning to future lighthouse-builders. Still more recently, the lighthouse on the Needles Down (also in the Isle of Wight), which for years had pointed out the dangerous character of the western entrance to the Solent, has been abandoned on account of the mists so frequently obscuring its lustre; and a new lighthouse has been erected on the outermost of the celebrated Needle Rocks, in a position of far greater utility.