What is holme `s signal?How it can beuse as a signal and tell the reaction involve in this process?

Containers which have a perforated bottom and a hole at the top are filled with calcium phosphide and calcium carbide. These are thrown into the sea. Water enters the container through the bottom and reacts with calcium carbide and calcium phosphide to give acetylene and phosphine. Phosphine gets ignited spontaneously as it comes in contact with air and also ignites acetylene. Hence, a bright red flame is produced which is accompanied by huge smoke due to burning of phosphine. This serves as a signal to approaching ships.

Reaction:

Ca3P2 + 6H2O --à 2PH3 + 3Ca(OH)2

 Calcium phosphate + water -à Phosphine +calcium hydroxide.