Isaac Watts (1674-1748) was dissenting minister, writing a generation after Tate and Brady. He put most of the psalms into metre, providing several versions of some of them. He also wrote a large number of paraphrases of other extracts of scripture and hymns. From childhood he seems to have had a remarkable ability to versify almost spontaneously.
Watts was much more relaxed than the others about fidelity to the original. His style is also more fluid. Many of his psalms are more like paraphrases, or even hymns inspired by a psalm. Other psalters very occasionally ‘christianise’ the Old Testament text. For example, the OV Psalm 2, for ‘against the Lord, and against his Anointed’, has ‘Against the Lord and Christ his Son whom he among us sent’. This, though, is unusual. Watts was far readier to do this or even to include thoughts which he felt suitable for christians even though they could hardly be in the original.
Thus several hundred years later at Ephesians 4:8, St Paul quotes Psalm 68:18 on Jesus’ ascension, ‘Thou art gone up on high, thou hast led captivity captive …’. Inspired by this, Watt’s expands Psalm 68:17-18 into four verses, headed ‘Christ’s Ascension, and the Gift of the Spirit’, and includes a stanza:
Rais’d by his Father to the throne,
he sent the promis’d Spirit down,
With gifts and grace for rebel men,
that God might dwell on earth again.
For Psalm 67 Watts renders ‘O be joyful in God all ye lands : sing praises unto the honour of his Name, make his praise to be glorious’ as:
Shine, mighty God, on Britain shine
with beams of heavenly grace:
Reveal thy pow’r thro’ all our coasts,
and shew thy smiling face.
This flexibility is a difference of approach that one either accepts or does not. I would imagine that most people would not regard the last example as acceptable.
Sometimes it works very well. Watts’ Psalm 90 ‘O God our help in ages past’ is still in every hymn book, though shortened, and altered. The original starts, ‘Our God, our help in ages past‘. ‘Joy to the world; the Lord is come’ is Watts’ version of part of Psalm 98. The hymn (very popular in west gallery circles as New Jerusalem) ‘Lo, what a glorious sight appears’ is his version of Revelation 21:1-4. The excellent hymn ‘Jesus shall reign, where’er the sun’ is actually Watts’ version of part of Psalm 72. Excellent it may be as a hymn, but it would really be wrong to describe it as a metrical version of that psalm, or to sing it in lieu of a psalm. It should only be described as a hymn inspired by the psalm rather than a metrical version of the psalm itself.