ART. 10.  A vessel which is being overtaken by another shall show from
  her stern to such last-mentioned vessel a white light or a flare-up
  light.

The white light required to be shown by this article may be fixed and carried in a lantern, but in such case the lantern shall be so constructed, fitted, and screened that it shall throw an unbroken light over an arc of the horizon of 12 points of the compass—­namely, for 6 points from right aft on each side of the vessel—­so as to be visible at a distance of at least 1 mile.  Such light shall be carried as nearly as practicable on the same level as the side lights.

ART. 11.  A vessel under 150 feet in length when at anchor shall carry forward, where it can best be seen, but at a height not exceeding 20 feet above the hull, a white light in a lantern so constructed as to show a clear, uniform, and unbroken light visible all around the horizon at a distance of at least 1 mile.

A vessel of 150 feet or upward in length when at anchor shall carry in the forward part of the vessel, at a height of not less than 20 and not exceeding 40 feet above the hull, one such light, and at or near the stern of the vessel, and at such a height that it shall be not less than 15 feet lower than the forward light, another such light.

  The length of a vessel shall be deemed to be the length appearing in her
  certificate of registry.

  A vessel aground in or near a fairway shall carry the above light or
  lights and the two red lights prescribed by article 4 (a).

ART. 12.  Every vessel may, if necessary in order to attract attention, in addition to the lights which she is by these rules required to carry, show a flare-up light or use any detonating signal that can not be mistaken for a distress signal.

ART. 13.  Nothing in these rules shall interfere with the operation of any special rules made by the government of any nation with respect to additional station and signal lights for two or more ships of war or for vessels sailing under convoy, or with the exhibition of recognition signals adopted by shipowners which have been authorized by their respective governments and duly registered and published.

  ART. 14.  A steam vessel proceeding under sail only, but having her
  funnel up, shall carry in daytime forward, where it can best be seen,
  one black ball or shape 2 feet in diameter.

  SOUND SIGNALS FOR FOG, ETC.

  ART. 15.  All signals prescribed by this article for vessels under way
  shall be given—­

  1.  By “steam vessels,” on the whistle or siren.

  2.  By “sailing vessels” and “vessels towed,” on the fog horn.

  The words “prolonged blast” used in this article shall mean a blast of
  from four to six seconds’ duration.