Like much of Meville, this is not an easy passage to read due to the ambiguity in punctuation and length. In his description of The Spouter-Inn, Melville describes several of the implements used in whale craft at the time hung on a wall.
One such implement Melville describes as being
having a crescent shaped blade
with a vast handle sweeping round
with a large, extensive handle all around
sweeping -> extensive
round -> around
like the segments made in the new-mown grass
like stalk segments in newly cut hay
he compares the general shape to a cut hay stalk with its top curled over
by a long-armed mower
by a scythe
Such an tool would have probably been used to cut the blubber of a whale.
Something like second from the left (below)