§ 3. When any muscular or other physical effort is required it is pretty evident that an increased duration or a greater continuity in the slighter effort may tax the body quite as severely as the less frequent or constant application of a much greater bodily force. There can be no question but that in a competitive industrial society there exists a tendency to compensate for any saving of hard muscular, or other physical effort afforded by the intervention of machinery in two ways: first, by "forcing the pace"--_i.e._, compelling the worker to attend more machines or to work more rapidly, thus increasing the strain, if not upon the muscles, then upon the nerves; secondly, by extending the hours of labour. A lighter form of labour spread over an increased period of time, or an increased number of minor muscular exertions substituted for a smaller number of heavier exertions within the same period of time, may of course amount to an increased tax upon the vital energy. It is not disputed that a general result of the factory system has been to increase the average length of the working day, if we take under our survey the whole area of machine-production in modern industrial communities. This is only in part attributable to the fact that workers can be induced to sell the same daily output of physical energy as before, while in many cases a longer time is required for its expenditure. Another influence of equal potency is the economy of machinery effected by working longer hours. It is the combined operation of these two forces that has lengthened the average working day. Certain subsidiary influences, however, also deserve notice, especially the introduction of cheap illuminants. Before the cheap provision of gas, the working time was generally limited by daylight. Not until the first decade of this century was gas introduced into cotton-mills, and another generation elapsed before it passed in general use in manufactories and retail shops.[205] Now a portion of nature's rest has been annexed to the working day. There are, of course, powerful social forces making for a curtailment of the working day, and these forces are in many industries powerfully though indirectly aided by machinery. Perhaps it would be right to say that machinery develops two antagonistic tendencies as regards the length of the working day. Its most direct economic influence favours an extension of the working hours, for machinery untired, wasting power by idleness, favours continuous work. But when the growing pace and complexity of highly-organised machinery taxes human energy with increasing severity, and compresses an increased human effort within a given time, a certain net advantage in limiting the working day for an individual begins to emerge, and it becomes increasingly advantageous to work the machinery for shorter hours, or, where possible, to apply "shifts" of workers.[206]