Dr Ian Plummer

http://www.oxfordcroquet.com/laws/6th/7.asp
Oxford Croquet

7. OUTSIDE AGENCIES

1. DEFINITION Subject to Law 7(b), an outside agency is any agency unconnected with the game. Examples include animals, spectators, a referee other than the players, the players or equipment from another game, a ball in hand, a ball not in play, a clip not attached to a hoop or the peg, the peg extension when not attached to the peg and other stray objects.
2. EXCLUSIONS Neither weather nor, except in exceptional circumstances dealt with under Law 55, loose impediments are outside agencies.
3. INTERFERENCE An outside agency should be moved or removed if it might affect play (see also Law 33(b) (interference by an outside agency) and Law 34(b) (fixed obstacles)).