Sentence:  The composer so ordered it, that the king's part should be one holding note, in a pitch proper for a Contratenor, for that was the king's voice. Nor was he inattentive to other particulars, for he contrived his own part, which was the Bass, in such a manner, that every other note he sung was an octave to that of the king, which prevented his majesty from deviating from that single note which he was to <WSD>intonate</WSD>. 
Meaning of the word marked with <WSD> tags:  To intone or vocalize (musical notes); to sound the tones of the musical scale; to practise the sol-fa.<ref>John S. Adams, "Adams' New Musical Dictionary", New York: S.T. Gordon, 1865, p.(nbsp)121: “Intonate. To sound the tones of the scale; to practise solmization; to read in a musical manner.”[https://catalog.hathitrust.org/Record/100695670]</ref>