A–––B–C. A–––B––C. A–––B–––C.

representing 3 different working-days of 7, 9, and 12 hours. The extension B—–C of the line A—–B represents the length of the surplus-labour. As the working-day is A—–B + B—–C or A—–C, it varies with the variable quantity B—–C. Since A—–B is constant, the ratio of B—–C to A—–B can always be calculated. In working-day I, it is 1/6, in working-day II, 3/6, in working day III 6/6 of A—–B. Since further the ratio (surplus working-time)/(necessary working-time), determines the rate of the surplus-value, the latter is given by the ratio of B—-C to A—-B. It amounts in the 3 different working-days respectively to 16 2/3, 50 and 100 per cent. On the other hand, the rate of surplus-value alone would not give us the extent of the working-day. If this rate, e.g., were 100 per cent., the working-day might be of 8, 10, 12, or more hours. It would indicate that the 2 constituent parts of the working-day, necessary-labour and surplus-labour time, were equal in extent, but not how long each of these two constituent parts was.

The working-day is thus not a constant, but a variable quantity. One of its parts, certainly, is determined by the working-time required for the reproduction of the labour-power of the labourer himself. But its total amount varies with the duration of the surplus-labour. The working-day is, therefore, determinable, but is, per se, indeterminate.

Although the working-day is not a fixed, but a fluent quantity, it can, on the other hand, only vary within certain limits. The minimum limit is, however, not determinable; of course, if we make the extension line B—‑C or the surplus-labour = 0, we have a minimum limit, i.e., the part of the day which the labourer must necessarily work for his own maintenance. On the basis of capitalist production, however, this necessary labour can form a part only of the working-day; the working-day itself can never be reduced to this minimum. On the other hand, the working-day has a maximum limit. It cannot be prolonged beyond a certain point.

This maximum limit is conditioned by two things. First, by the physical bounds of labour-power.Within the 24 hours of the natural day a man can expend only a definite quantity of his vital force. A horse, in like manner, can only work from day to day, 8 hours. During part of the day this force must rest, sleep; during another part the man has to satisfy other physical needs, to feed, wash, and clothe himself. Besides these purely physical limitations, the extension of the working-day encounters moral ones. The labourer needs time for satisfying his intellectual and social wants, the extent and number of which are conditioned by the general state of social advancement. The variation of the working-day fluctuates, therefore, within physical and social bounds. But both these limiting conditions are of a very elastic nature, and allow the greatest latitude. So we find working-days of 8, 10, 12, 14, 16, 18 hours, i.e., of the most different lengths.