So far in this chapter we have dealt solely with the development of the sea-transportation of oil itself; but even larger vistas are opened when we come to its growing relation to all forms of maritime commerce and naval activity. This arises from the rapidly increasing use of oil as a marine fuel. In that respect it holds very high potentialities for America’s seaborne trade. The oil tankers we have been describing are oil burning, and the same system is being applied to many other types of vessels which constitute the arteries of the world’s trade. Until quite recently the supremacy of Great Britain in maritime commerce was in a considerable measure due to her plentiful supplies of bunker coal obtainable at low cost in ports like Swansea, Wales. But the definite advantages of oil as a fuel for the navigation of steamships are changing the whole maritime equilibrium. As an English writer has said, the position that oil has captured for itself in this respect has been fairly won on its merits. Oil fuel has one and a half times the heating power of steam coal, so that weight for weight carried, the radius of action is extended fifty per cent. A vessel equipped with a modern internal combustion engine consuming fuel oil may make a voyage of fifty-seven days without replenishment, whereas the same vessel operated by the old type of coal-fuelled steam engine would be obliged to re-fill its bunkers at the end of fifteen days. In 1912 an Oil Congress was held in London, England, when statistics were presented containing a comparison between coal and fuel oil on the great Cunard Liner the _Mauretania_. It was shown that for the round trip from Liverpool to New York and back there would be a saving of at least 5000 tons of fuel and that the force of stokers required could be reduced from 300 to 30 men working under much less difficult conditions. The resultant increase in available space for cargo and passengers is of enormous importance to ship-owners. The relative values of oil and coal for marine use are not limited to the superiority of oil engines over the old-fashioned steam engines. The caloric or steam-raising power of oil is so much greater than that of coal as to produce a fifty per cent. superiority. Another factor is that of cleanliness. Coal is not merely bulky and prolific of many inconveniences in the confined space of a ship, but it is unquestionably dirty, as every harbour bears ample testimony. Oil is clean, smokeless and leaves no ashes and clinkers. It can be pumped on board from a tender while both ships are making considerable speed. The late war furnished innumerable demonstrations of the superiority of oil as a source of motive power at sea, which will be presently dealt with; as an aid to peaceful commerce its influence during the next few years is certain to be revolutionary and incalculable in its benefits.