“In point of fact, however, whether a man works three days of the week for himself on his own field and three days for nothing on the estate of his lord, or whether he works in the factory or the workshop six hours daily for himself and six for his employer, comes to the same, although in the latter case the paid and unpaid portions of labour are inseparably mixed up with each other, and the nature of the whole transaction is completely masked by the intervention of a contract and the pay received at the end of the week. The gratuitous labour appears to be voluntarily given in the one instance, and to be compulsory in the other. That makes all the difference.”