given the productiveness of labour and its normal intensity, the rate of surplus value can be raised only by the actual prolongation of the working day; on the other hand, given the length of the working day, that rise can be effected only by a change in the relative magnitudes of the components of the working day, viz., necessary labour and surplus labour; a change which, if the wages are not to fall below the value of labour power, presupposes a change either in the productiveness or in the intensity of the labour.