Of the difficulties encountered in the construction of lighthouse towers on outlying rocks much has been written. The historic Eddystone, for example, has quite a voluminous literature of its own. Of the light itself, however, much less is known. It will be interesting first to note the different purposes for which a light may be required, and then see how the apparatus of the lighthouse is made to serve these purposes. There is the "making" light, perched, if possible, upon some high eminence, deriving its name from the fact that the sailor sights it as he is "making" the land. Vessels approaching England from the south-west by night first see the light at the Lizard. The transatlantic vessels know they are approaching land by catching sight of the Fastnet Rock light off the coast of Ireland. Cape Race light serves in the same way for those about to enter the St Lawrence and Navesink for the entrance to New York harbour. All such as these have to be of the greatest power practicable, so that they may be visible not only at the longest possible distance, but also under unfavourable conditions, such as haze and slight fog. No light, of course, can penetrate thick fog, but in light fog and haze a powerful light can be seen at considerable distances. For the same reason these lights must be high up, or the curvature of the ocean's surface will limit their range. A light elevated 100 feet above the sea-level will be visible nearly 16 miles away, but if only 50 feet up it will be invisible at 13 miles. To be seen 40 miles away it must be as high as 1000 feet. But then again height is in some cases a disadvantage, for sometimes fog hovers a little distance above the sea, while below it the air is clear, and the higher a light may be the more likely is it to have its lantern immersed in a floating cloud of fog. Many readers familiar with the south coast of Britain will remember that the light which used to show on the summit of Beachy Head is there no more, but has been replaced by a tower at the foot of the cliffs, the reason being that it may be below the clouds of fog which are prevalent at that point.