On the other hand, it is evident that the duration of the surplus-labour
 is given, when the length of the working day, and the value of labour-power,
 are given. The value of labour-power, i.e., the labour-time requisite to
 produce labour-power, determines the labour-time necessary for the reproduction
 of that value. If one working-hour be embodied in sixpence, and the value
 of a day’s labour-power be five shillings, the labourer must work 10 hours
 a day, in order to replace the value paid by capital for his labour-power,
 or to produce an equivalent for the value of his daily necessary means
 of subsistence. Given the value of these means of subsistence, the value
 of his labour-power is given; [1] and given the value
 of his labour-power, the duration of his necessary labour-time is given.
 The duration of the surplus-labour, however, is arrived at, by subtracting
 the necessary labour-time from the total working day. Ten hours subtracted
 from twelve, leave two, and it is not easy to see, how, under the given
 conditions, the surplus-labour can possibly be prolonged beyond two hours.
 No doubt, the capitalist can, instead of five shillings, pay the labourer
 four shillings and sixpence or even less. For the reproduction of this
 value of four shillings and sixpence, nine hours’ labour-time would suffice;
 and consequently three hours of surplus-labour, instead of two, would accrue
 to the capitalist, and the surplus-value would rise from one shilling to
 eighteen-pence. This result, however, would be obtained only by lowering
 the wages of the labourer below the value of his labour-power. With the
 four shillings and sixpence which he produces in nine hours, he commands
 one-tenth less of the necessaries of life than before, and consequently
 the proper reproduction of his labour-power is crippled.298 The surplus-labour
 would in this case be prolonged only by an overstepping of its normal limits;
 its domain would be extended only by a usurpation of part of the domain
 of necessary labour-time. Despite the important part which this method
 plays in actual practice, we are excluded from considering it in this place,
 by our assumption, that all commodities, including labour-power, are bought
 and sold at their full value. Granted this, it follows that the labour-time
 necessary for the production of labour-power, or for the reproduction of
 its value, cannot be lessened by a fall in the labourer’s wages below the
 value of his labour-power, but only by a fall in this value itself. Given
 the length of the working day, the prolongation of the surplus-labour must
 of necessity originate in the curtailment of the necessary labour-time;
 the latter cannot arise from the former. In the example we have taken,
 it is necessary that the value of labour-power should actually fall by
 one-tenth, in order that the necessary labour-time may be diminished by
 one-tenth, i.e., from ten hours to nine, and in order that the surplus
 labour may consequently be prolonged from two hours to three.