which is dead ahead.  They are screened so as not to be visible across the bow and they must be intense enough to be visible two miles ahead.  The masthead light is carried on the foremast and the range-light on the mainmast, at an elevation fifteen feet higher than the former.  The range-light emits light toward all points of the compass and must be intense enough to be seen at a distance of three miles.  The stern light is similar to the masthead, but its light must not be visible forward of the beam.  When a vessel is towing another it must display two or three lights in a vertical line with the masthead light and similar to it.  The lights are spaced about six feet apart, and two extra ones indicate a short tow and three a long one.  A vessel over a hundred and fifty feet long when at anchor is required to display a white light forward and aft, each visible around the entire horizon.  These and many other specifications indicate how artificial light informs the mariner and makes for order in shipping.  Without artificial light the waterways would be trackless and chaos would reign.

The distress signals of a vessel are rockets, but any burning flame also serves if rockets are unavailable.  Fireworks were known many centuries ago and doubtless the possibilities of signaling by means of rockets have long been recognized.  An early instance of scientific interest in rockets and their usefulness is that of Benjamin Robins in 1749.  While he was witnessing a display of fireworks in London it occurred to him that it would be of interest to measure the height to which the rockets ascended and to determine the ranges at which they were visible.  His measurements indicated that the rockets ascended usually to a height of 440 yards, but some of them attained altitudes as high as 615 yards.  He then had some special ones made and despatched letters to friends in three different localities, at distances as great as 50 miles, asking them to observe at a certain time, when the rockets were to be sent up in the outskirts of London.  Some of these rockets rose to altitudes as great as 600 yards and were distinctly seen by observers 38 miles away.  Later he made rockets which ascended as high as 1200 yards and concluded that this was a practical means of signaling.  Since that time and especially during the recent war, rockets have served well in signaling messages.

The self-propelled rockets have not been altered in essential features since the remote centuries when the Chinese first used them in celebrations.  A cylindrical shell is mounted on a wooden stick and when the powder in the shell burns the hot gases are ejected so violently downward that the reaction drives the shell upward.  At a certain point in the air, various signals burst forth, which vary in character and color.  One of the advantages of the rocket is that it contains within itself the force of propulsion; that is, no gun is necessary to project it.  The illuminating compounds and various details are similar to those of the illuminating shells described in another chapter.