In the following rules every steam vessel which is under sail and not
  under steam is to be considered a sailing vessel, and every vessel under
  steam, whether under sail or not, is to be considered a steam vessel.

  The words “steam vessel” shall include any vessel propelled by
  machinery.

  A vessel is “under way” within the meaning of these rules when she is
  not at anchor or made fast to the shore or aground.

  RULES CONCERNING LIGHTS, ETC.

  The word “visible” in these rules when applied to lights shall mean
  visible on a dark night with a clear atmosphere.

  ARTICLE 1.  The rules concerning lights shall be complied with in all
  weathers from sunset to sunrise, and during such time no other lights
  which may be mistaken for the prescribed lights shall be exhibited.

  ART. 2.  A steam vessel when under way shall carry—­

(a) On or in front of the foremast, or if a vessel without a foremast, then in the fore part of the vessel, at a height above the hull of not less than 20 feet, and if the breadth of the vessel exceeds 20 feet, then at a height above the hull not less than such breadth, so, however, that the light need not be carried at a greater height above the hull than 40 feet, a bright white light so constructed as to show an unbroken light over an arc of the horizon of 20 points of the compass, so fixed as to throw the light 10 points on each side of the vessel—­namely, from right ahead to 2 points abaft the beam on either side—­and of such a character as to be visible at a distance of at least 5 miles.

(b) On the starboard side a green light so constructed as to show an unbroken light over an arc of the horizon of 10 points of the compass, so fixed as to throw the light from right ahead to 2 points abaft the beam on the starboard side, and of such a character as to be visible at a distance of at least 2 miles.

(c) On the port side a red light so constructed as to show an unbroken light over an arc of the horizon of 10 points of the compass, so fixed as to throw the light from right ahead to 2 points abaft the beam on the port side, and of such a character as to be visible at a distance of at least 2 miles.

  (d) The said green and red side lights shall be fitted with
  inboard screens projecting at least 3 feet forward from the light, so as
  to prevent these lights from being seen across the bow.

(e) A steam vessel when under way may carry an additional white light similar in construction to the light mentioned in subdivision (a).  These two lights shall be so placed in line with the keel that one shall be at least 15 feet higher than the other, and in such a position with reference to each other that the lower light shall be forward of the upper one.  The vertical distance between