{{explanation}}
1s & 3s start on opp sides

1-8
: All set, 1s+2s also 3s+4s circle 4H round to left opening out in middle
  to 1s facing 2s & 3s facing 4s, all set
9-16
: All petronella turn into lines across (Men BtoB facing other partner) &
  all set, all petronella turn onto sides & set.  2(1)4(3)
17-24
: All dance Ladies' Chain & 1s end facing down, 4s facing up
25-32
: 1s+4s dance 1/2 R&L, all set & cross RH. (2)4(1)3