The fact that radius has been sought by Sea Power in all ages has already been remarked upon briefly. To increase their radius the early Egyptians and Greeks supplemented the oar by the sail. At a later period the sail supplanted the oar, because it gave an increased radius, and, finally, steam did not replace the sail until the use of it conferred a radius at least sufficient for all practical needs. The early steamers were masted so that radius should in no way be reduced by the limitations of bunker capacity; the masted warship though a wretched sailer only died out when it became clear that by the establishment of coaling stations and increased bunker capacity there should be no loss of needful radius to counterbalance the gain which steam conferred in other directions. Here, then, appears a principle which, having controlled all the past, may confidently be expected to affect the future.
As regards the immediate future we have seen the law in imperfect operation in the adoption of water-tube boilers, all types of which increase effective radius by conferring the ability to raise steam quickly and, in most types, to maintain high powers over extended periods. These two facts made the abandonment of the old-type cylindrical boilers certain; and those who fought for the retention of cylindrical clearly ignored the trend of history throughout all time.
As things are, the universal adoption of the water-tube boiler must be said to rest chiefly on its advantages