Whenever any person convicted of or charged with an offence, being in lawful custody for that offence, escapes from such custody, or whenever a public servant, in the exercise of the lawful powers of such public servant, orders a certain person to be apprehended for an offence, whoever, knowing of such escape or order for apprehension, harbours or conceals that person with the intention of preventing him from being apprehended, shall be punished in the manner following, that is to say: