At the round earth's imagined corners, blow
Your trumpets, angels, and arise, arise
From death, you numberless infinities
Of souls, and to your scattered bodies go,
All whom the flood did, and fire shall, o'erthrow,
All whom war, dearth, age, agues, tyrannies,
Despair, law, chance, hath slain, and you whose eyes,
Shall behold God, and never taste death's woe.
But let them sleep, Lord, and me mourn a space;
For, if above all these, my sins abound,
'Tis late to ask abundance of thy grace,
When we are there. Here on this lowly ground,
At this point, the volta, there is a pause or  caesura , and the new approach is introduced with the conjunction ‘but’. 
 The tone is quieter, less dramatic, slower in pace — the effect of the long vowels in ‘sleep’, ‘Lord’, ‘mourn’ and ‘space.’ 
 Donne is saying he wants sinners to sleep longer, to have more time before the terrible Judgment. For his part he needs to ‘mourn’, maybe contemplate and come to terms with and repent his sins.
At the round earths imagined corners Holy Sonnet 7