4. We thus see that not only are the requisite repairs most extensive and costly, but of such magnitude as to greatly reduce the earnings of any class of steam vessels. But this is not the last costly consequence of mail speed. It requires more cautious watchfulness of the engines, the boilers, the deck, and of every possible department of the navigation, even including pilotage. It requires also more promptness and dispatch in every movement, and hence a much larger aggregate number of men. More men are necessary to keep up high fires; twice as many men are necessary to pass twice as much coal; twice as many engineers as under other circumstances are necessary for the faithful working of the engines, and any accidents and repairs which are indispensable on the ocean; and a larger number of sailors and officers is necessary to all of the prompt movements required of the mail steamer. The Havre mail steamers, the "Arago" and "Fulton," never carry less than six engineers each, although they could be run across the ocean with three under a hard working system. But this number insures the greater safety of the ship under ordinary circumstances, and is absolutely necessary in any case of accident and danger. It is the same case with the firemen. When, in a heavy storm, the fire department may be imperfectly manned, the ship has taken one of the first chances for rendering the engines inefficient, and being finally lost. And all of these extra and indispensable _employees_ make an extra drain on the income of the ship, and add to the extreme costliness of a high adequate mail speed.