John Wesley’s NotesPsalms 137

Sat - The usual posture of mourners.

Harps - Harps are here put for all instruments of musick.

A song - Such songs as you used to sing in the temple of Zion.

The Lord's - Those songs which were appointed by God to be sung only in his service.

If - If I do not value Jerusalem's prosperity more than all other delights.

The day - In the time of its destruction.

Happy - As being God's instrument to vindicate his honour, and execute his just judgments.