“Well,” Mervyn continued, “it seems that he has had his old school chum, Tom Wilson, the engineer, staying with him at Hilton Manor for several months, and between them they have managed to construct a submarine, which, if it but answer their expectations, will prove the very thing I have been waiting for all these years. This is Garth’s description of his craft,” and, extracting a letter from the depths of a bulky note-book, Mervyn read as follows: “Total length, three hundred and fifty feet; beam, fifty feet; torpedo-shaped, with turret or wheelhouse, from which the vessel is governed, in centre of deck. Tanks for submerging or raising; air reservoirs for supply whilst beneath the surface; liquid air engines, a patent of Wilson’s, maximum speed of which is forty-five knots per hour upon the surface, and thirty submerged.” “Whew!” The professor’s companion whistled in his astonishment at this last statement. “Liquid air engines!” he said. “Why, I always thought that liquid air was a powerful explosive agent?” “True,” returned Mervyn; “but you must also remember that steam becomes an explosive when compressed, as witness the recent boiler explosion, so that is no argument against the use of liquid air as a propelling power.” “But I don’t quite see----” the baronet began in a puzzled tone. “Let me try to make it clear to you,” interrupted Mervyn. “Though but eighteen, young Tom Wilson is already recognised as an authority on the subject of liquid air and its capabilities as a propelling agent. As you will recollect, his father was a famous engineer, and the family talent appears to have descended to the lad. “Ever since he left school Tom has been working on his engines, lack of funds alone preventing him from perfecting them before now. With financial aid from Garth, however, he has at last been enabled to complete them, and I give you my word they are the finest set of engines I have ever been privileged to examine. “The huge boiler is somewhat similar in shape to that of an ordinary marine engine, but is much larger, and contains a number of immense tubes, in which is stored the liquefied air. From these the stuff works direct upon the powerful cylinders. Heat, of course, is entirely unnecessary; in fact, it would shatter the whole affair to atoms, liquid air being many degrees colder than ice. “The first two gallons of the stuff cost Garth six hundred pounds to make; but there the expense ends, the engines drawing their own supplies from the air as they work.” “Wonderful!” Seymour cried; “and the vessel does forty-five knots to the hour, you say? What will the world think of it when the news becomes public?” “The news will never become public,” retorted the scientist, “if we can avoid it. Garth has taken the greatest care to prevent the facts leaking out. All his workers are picked men, and have been sworn to secrecy with regard to the nature of the vessel upon which they are engaged.”