Sentence:  When the 'Canandaigua' got astern, and was lying athwart, of the 'Housatonic,' about four ship lengths off, while I was in the fore rigging, I saw a <WSD>blue light</WSD> on the water just ahead of the 'Canandaigua,' and on the starboard quarter of the 'Housatonic.' 
Meaning of the word marked with <WSD> tags:  A mixture of chemicals (including nitre, sulfur and antimony) used in eighteenth and nineteenth centuries for pyrotechnics, night-time signaling and general illumination.