One keeper and three assistants guard the light over Minot's Ledge. Three miles away across the sea, now blue and smiling, now black and wrathful, they can see the little group of dwellings on the Cohasset shore which the Government provides for them, and which shelter their families. The term of duty on the rocks is two weeks; at the end of each fortnight two happy men go ashore and two grumpy ones come off; that is, if the weather permits a landing, for keepers have been stormbound for as long as seven weeks. The routine of duty is much the same in all of the lighthouses. By night there must be unceasing watch kept of the great revolving light; and, if there be other lights within reach of the keeper's glass, a watch must be kept on them as well, and any eclipse, however brief, must be noted in the lighthouse log. By day the lens must be rubbed laboriously with a dry cloth until it shines like the facets of a diamond. Not at all like the lens we are familiar with in telescopes and cameras is this scientifically contrived device. It is built up of planes and prisms of the finest flint glass, cut and assembled according to abtruse mathematical calculations so as to gather the rays of light from the great sperm-oil lamp into parallel rays, a solid beam, which, in the case of Minot's Ledge light, pierces the night to a distance of fifteen miles. On foggy days, too, the keepers must toll the fog-bell, or, if the light be on the mainland, operate the steam siren which sends its hoarse bellow booming through the gray mist to the alert ears of the sailor miles away. The regulations do not prescribe that the keeper of a light shall hold himself ready to go to the assistance of castaways or of wrecked vessels; but, as a matter of fact, not a few of the most heroic rescues in the history of the coast have been performed by light-keepers. In the number of lives saved a woman--Ida Lewis, the keeper of the Limerock Light in Newport Harbor--leads all the rest. But there is hardly any light so placed that a boat can be launched that has not a story to tell of brave men putting out in frail boats in the teeth of a roaring gale to bring in some exhausted castaways, to carry a line to some stranded ship, or to guide some imperiled pleasure-seekers to safety.