The Chain Saw

A Kerryman got a job as a lumberjack, but try as he might, he couldn't meet his quota of fifty trees a day. By chance he saw an ad. in a shop window for chain saws 'guaranteed to fell 60 trees a day'. So he bought one, but the best he could manage was forty trees a day. So he took it back to the shop and complained that there must be something wrong with it.
'Let me look at it', said the man in the shop. So he took the chain saw and switched it on.
'What's that noise?' said the Kerryman.