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

1-8
: 1L+2M also 3M+4L set & cast 1 place, (1L & 3M down, 2M & 4L up) into
  line up/down centre of set. 2M+1L also 4L+3M turn LH 1.1/2 to face
  partners
9-16
: 1s+2s also 3s+4s dance diagonal reels of 4 across (on 1st corner
  diagonals) to end all back in original places 1 2 (3) (4)
17-24
: 2s+3s set & Petronella-in-tandem twice to end 1 3 (2) (4)
25-32
: 1s+3s also 2s+4s dance 'Double Knot', figure similar to normal 2 couple
  Knot but at the end of 2nd bar 1s+3s face down (between middle of set
  & Men's side) as 2s+4s face up (between middle of set & Ladies' side),
  3s followed by 1s dance down to 4th/3rd places as 2s+4s dance up to
  1st/2nd places & all turn LH to places 2 4 (1) (3)