{{explanation}}
2 chords: 2nd chord 3s & 4s cross to opp sides

1-8
: 1s+2s also 3s+4s set facing up/down on sides & Petronella turn into
  lines of 4 across; all dance 1/2 reels of 4 across
9-16
: 1s+2s also 3s+4s set facing across & Petronella turn to other sides (1)
  (2) 3 4; all dance 1/2 reels of 4 on sides.  4 3 (2) (1)
17-24
: 4s+1s (end couples) chase clockwise 1/2 way to opposite ends; 1s & 4s
  birl giving RH (elbow grip).  1 3 (2) (4)
25-32
: All set, 1s+3s (at top) also 2s+4s (at bottom) dance 1/2 RH across; all
  set & cross RH with partner.  3 1 (4) (2)