The degree of exploitation of labour, the appropriation of surplus-labour and surplus-value, is raised notably by lengthening the working-day and intensifying labour. These two points have been comprehensively treated in Book I as incidental to the production of absolute and relative surplus-value. There are many ways of intensifying labour which imply an increase of constant, as compared to variable, capital, and hence a fall in the rate of profit, such as compelling a labourer to operate a larger number of machines. In such cases — and in most procedures serving the production of relative surplus-values — the same causes which increase the rate of surplus-value, may also, from the standpoint of given quantities of invested total capital, involve a fall in the mass of surplus-value. But there are other aspects of intensification, such as the greater velocities of machinery, which consume more raw material in the same time, but, so far as the fixed capital is concerned, wear out the machinery so much faster, and yet do not in any way affect the relation of its value to the price of the labour which sets it in motion. But notably, it is prolongation of the working-day, this invention of modern industry, which increases the mass of appropriated surplus-labour without essentially altering the proportion of the employed labour-power to the constant capital set in motion by it, and which rather tends to reduce this capital relatively. Moreover, it has already been demonstrated — and this constitutes the real secret of the tendency of the rate of profit to fall — that the manipulations to produce relative surplus-value amount, on the whole, to transforming as much as possible of a certain quantity of labour into surplus-value, on the one hand, and employing as little labour as possible in proportion to the invested capital, on the other, so that the same reasons which permit raising the intensity of exploitation rule out exploiting the same quantity of labour as before by the same capital. These are the counteracting tendencies, which, while effecting a rise in the rate of surplus-value, also tend to decrease the mass of surplus-value, and hence the rate of profit produced by a certain capital. Mention should also be made here of the widespread introduction of female and child labour, in so far as the whole family must now perform more surplus-labour for capital than before, even when the total amount of their wages increases, which is by no means always the case. — Everything that promotes the production of relative surplus-value by mere improvement in methods, as in agriculture, without altering the magnitude of the invested capital, has the same effect. The constant capital, it is true, does not, in such cases, increase in relation to the variable, inasmuch as we regard the variable capital as an index of the amount of labour-power employed, but the mass of the product does increase in proportion to the labour-power employed. The same occurs, if the productiveness of labour (no matter, whether its product goes into the labourer's consumption or into the elements of constant capital) is freed from hindrances in communications, from arbitrary or other restrictions which have become obstacles in the course of time; from fetters of all kinds, without directly affecting the ratio of variable to constant capital.