The utilization of the lightship, however, is not restricted by any means to marking shoals and sandbanks. Here and there are clusters of rocks obstructing the ocean highway, which from their extremely exposed character would offer the engineer a searching and expensive problem to solve, and which, accordingly, are protected by a floating light. But, taken on the whole, the lightship is used very sparingly. If it is at all possible to provide a permanent structure, even at an apparently prohibitive cost, upon a danger spot, this practice is followed in preference to the mooring of a light-vessel thereto. A masonry tower is stationary in its resistance to the assaults of the wildest tempest, but the lightship swings like a cork at the free end of a chain. At times it drags its anchors, and thereby unconsciously shifts its position, so that it may throw its light from some distance beyond the actual area of danger. Again, a lightship, although not costly in the first instance, is somewhat expensive to maintain. It cannot withstand the poundings of the waves and the force of the wind for long without developing some signs of weakness. It may ride over its reef or shoal for several years, but depreciation is sure to set in, so that at last it becomes too decrepit to be trusted. Moreover, the number of men required to man a lightship exceeds the force necessary to maintain a lighthouse. Lightships follow much the same general shape and construction the whole world over. There is very little opportunity to depart from well-tried lines; the experience of a century and more has indicated conclusively the form of hull, as regards both material and shape, best adapted to the peculiar work which has to be fulfilled. The modern lightship is essentially a British idea, the first floating beacon of this description having been built and placed in the mouth of the Thames as far back as 1713. From this small beginning, which virtually was an experiment, has grown the large fleet of light-vessels scattered all over the globe. The craft is sturdily built, and, although of clumsy appearance, is capable of withstanding the onslaughts of the fiercest gales. Internally it is made as snug as possible, but the opportunities in this direction are not very extensive, as the beacon is built primarily to protect ships and lives against accident, and comfort is necessarily made subordinate to reliability, durability, and serviceability.