Now I have prepared a few diagrams showing the advantage of various types of vessels in evading the submarine, and of these I shall treat immediately, as they illustrate the points of my contention perfectly. There was a time when everybody thought the earth was flat, but now I believe it is generally conceded that it is round. Every one knows that when the sun or moon sinks beneath the horizon it cannot be seen, neither can anything else which is below the horizon, so if the horizon intervenes between two distant observers they cannot see each other. Now by referring to our text-books we find that if an observer is stationed at a height of fifteen feet above the surface of the sea the horizon is five and one-eighths miles distant, so that if there were another observer stationed on the other side of the horizon at the same distance and height from the surface of the sea they could not see each other, as the surface of the earth or sea, being round, would stand up like a hill between them.
THE "DEUTSCHLAND"
By Courtesy of Motorship
The "Deutschland" was the first submarine cargo-carrier to cross the Atlantic Ocean. She was under the command of Captain Paul Koenig and proved the practicability of running the English blockade four times before war between Germany and the United States caused her owners to discontinue her sailings. Had war not come between the two countries, her German owners would undoubtedly have had submarine cargo-carrying vessels making weekly sailings between the United States and Germany.
The diagram shown herewith shows the distance of horizon in miles from 0 to two hundred feet elevation above the surface of the water.
I have drawn a sketch—in which the scale of distance is exaggerated in order to better illustrate my meaning—of the earth's surface to show the comparative visibility of vessels when seen from a military submarine, lying in wait, with periscope extended fifteen feet above water. Now take such ships as the Lusitania, shown in position No. 5 on the diagram, with her smoke-stacks extending over one hundred feet above the surface of the sea; their tops would appear above the horizon and become visible to a distant observer with a powerful glass, stationed at, say, fifteen feet above the surface, at a distance of about eighteen and three-eighths miles. Her smoke-stack would also become visible through a telescopic periscope, the object glass of which was extended fifteen feet above the surface, while men seated in a rowboat could not see each other because of the intervening "hill," so to speak, at a distance of four miles apart. If they were under water in a submarine they could not see each other at all unless they had the periscopes elevated above the surface. In that case it would not be possible for one periscope to see another at any considerable distance, because the periscope is such a small object, and vision through it does not compare with natural vision, owing to the fact that there is considerable loss of light in passing the image of external objects through lenses and prisms. Hence it has been found necessary to reduce the field of vision to about one-half that of natural vision to give the effect of true distance, and as soon as twilight falls it is practically useless. I have taken fifteen feet above the surface without the submarine's conning tower showing, for if her conning tower is shown above the surface she is in danger of being herself discovered.
From the above data we are able to determine the probability of being discovered. We take the case of the largest and fastest ocean liners, such as the Lusitania as one illustration. We will assume that the Lusitania is making her maximum speed of about twenty-five knots, which is about the maximum of speed yet attained in a large surface freight-and passenger-carrying ship, and from our scale of vision as applied to upper diagram No. 5 we see that her top works will become visible above the horizon at a distance of eighteen and three-eighths miles from the periscope of the submarine. The commander in the submarine, by using his range and direction finder with which all military submarines are fitted, finds the ship to be pursuing a course and speed that will cause her to pass probably within ten miles of the submarine station in about thirty-five minutes, which is too far off to attack by torpedo. Now, while submarines have a submerged speed of only about ten knots, the commander is quickly able to ascertain that he can intercept the twenty-five-knot boat by laying his own course at right angles to the approaching ship, and that, if the ship keeps her course and speed, in thirty-five minutes he can be within torpedo range, as will be seen by reference to this sketch (see diagram, position No. 5).
DIAGRAM TO ILLUSTRATE THE COMPARATIVE VISIBILITY AND CONSEQUENTLY THE COMPARATIVE SAFETY OF SURFACE SHIPS AND CARGO-CARRYING SUBMARINES