As used at present liquid fuel simply substitutes coal for heating the ordinary type of boiler. But there seems every reason to believe that in the near future some type of internal combustion engine will be perfected that will use the crude, cheap oil, as the finer and lighter oils are used in motors to-day. When this occurs the space-consuming boilers and furnaces used in ships at present will be replaced by compact machinery, quite as efficient, but occupying only a fraction of the space. Nor need we expect that the invention of some such type of engine will be long delayed, if we may judge by the rapid strides made in perfecting other internal combustion engines during the past few years. III SUBMARINE VESSELS The development of submarine vessels has been one of the slowest in the history of modern inventions. Submarine boats, using submarine torpedoes, were able to destroy ships a hundred years ago; and a little less than half a century ago naval vessels were destroyed in actual warfare by these boats. But curiously enough no vessel has ever been destroyed in actual warfare by a submarine boat since that time. Yet these boats are essentially war-vessels, and, with the exception of boats of the Lake type, of no use whatever for commercial purposes. Perhaps the explanation for this tardy development lies in the fact that until recent years naval men have not looked with favor upon this style of fighting craft. In Admiral Porter's book, written in 1878, he makes the statement that one of the reasons why they did not show more enthusiasm about the submarine made by Robert Fulton early in the nineteenth century, was that such boats "menaced the position of the naval men, whose calling would be gone did the little submarine boat supplant the battle-ship." We need not, however, depend upon this statement, made as it was three-quarters of a century after the demonstrations by Fulton, for there are many similar statements made at the time to be had at first hand. Thus Admiral Earl St. Vincent, when opposing the views of William Pitt, who had become enthusiastic over the possibilities of Fulton's submarines, is on record as having opposed such craft on the ground that by encouraging such development "he was laying the foundation which would do away with the navy." In 1802, M. St. Aubin wrote in this connection, "What will become of the navies, and where will sailors be found to man ships of war, when it is a physical certainty that they may at any time be blown into the air by diving boats, against which no human foresight can guard them?" Such opposition has undoubtedly tended to retard the progress of submarine navigation; but be the cause what it may, it has made slow and laborious work of it; and we are only now approaching a solution of the question that seemed almost within grasp a hundred years ago--before the days of steam or electricity. THE FIRST SUBMARINE