"ART. 15. All signals prescribed by this article for vessels under way shall be given—
"First. By 'steam vessels,' on the whistle or siren.
"Second. 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.
"A steam vessel shall be provided with an efficient whistle or siren, sounded by steam or some substitute for steam, so placed that the sound may not be intercepted by any obstruction, and with an efficient fog horn to be sounded by mechanical means, and also with an efficient bell. (In all cases where the rules require a bell to be used a drum may be substituted on board Turkish vessels or a gong where such articles are used on board small seagoing vessels.) A sailing vessel of 20 tons gross tonnage or upward shall be provided with a similar fog horn and bell.
"In fog, mist, falling snow, or heavy rain storms, whether by day or night, the signals described in this article shall be used as follows, namely:
"(a) A steam vessel having way upon her shall sound at intervals of not more than two minutes a prolonged blast.
"(b) A steam vessel under way, but stopped and having no way upon her, shall sound at intervals of not more than two minutes two prolonged blasts with an interval of about one second between.
"(c) A sailing vessel under way shall sound at intervals of not more than one minute, when on the starboard tack one blast, when on the port tack two blasts in succession, and when with the wind abaft the beam three blasts in succession.
"(d) A vessel when at anchor shall at intervals of not more than one minute ring the bell rapidly for about five seconds.