Above the limit of 10 or 11 hours the lengthening of labour time seems to diminish rather than to increase its aggregate productivity, and this explains why the 11 and 10 hours day, without any intervention from the State, has been so generally and successfully adopted by custom and contract. It is the general experience, as the Duesseldorf inspector notes in his report, that "those works in which the smallest amount of labour is performed, have as a rule the longest hours of labour; all attempts to increase the amount of labour at favourable periods of the market, by offering higher wages, whilst at the same time maintaining the long hours, have only attained a short-lived success, or have altogether failed; the same result is produced when in certain occupations the usually short hours of labour are prolonged in order to profit by the opportunity of a good market; it is only for the first few days that the increase in the amount of work executed corresponds to the increase in the hours of work, and the old level is quickly resumed; on the other hand, it is frequently affirmed by the managers that the capacity for work of our labourers is in no wise inferior to that of the English."[11] The legal 11 or 10 hours day would not be justified if custom and freedom of contract were sufficient to adjust the true proportions of working time. This however is not the case, and the legal working-day is therefore necessary in order to supplement the work of free self-protection. With regard to the voluntary adjustment of the duration of the working-day, we find that the 10 and 11 hours day already prevails in a large proportion of the German industries: as in Bremen, whence according to the factory report, only 33.8 per cent. of the adult labourers work beyond 10 hours, and only 3.8 per cent. beyond 11 hours, and in Berlin, where in 3,070 firms, 71,465 male labourers work for 10 hours and less; and the same is reported by other district inspectors. But side by side with this we find a longer and frequently a decidedly too long working-day, and nowhere does every firm adhere to the 10 or 11 hours day. Even in the Lower Rhine Provinces the 12 hours working-day is in force in the smelting houses (Hitze). In Saxony the same number of hours obtains, as a rule, in textile industries, although many manufacturers would prefer the 10 hours day, if all competitors would adopt it. In Bavaria and Baden the 11 to 12 hours working-day prevails widely. In certain separate kinds of work, as in mills and brick kilns, the working hours are even longer.