muscles exercised a capacity for enduring fatigue to a much greater degree than they could support under other circumstances. A similar result seems to take place in any change of mental exertion; the attention bestowed on the new subject not being so perfect at first as it becomes after some exercise. The employment of different tools in the successive processes, is another cause of the loss of time in changing from one operation to another. If these tools are simple, and the change is not frequent, the loss of time is not considerable; but in many processes of the arts, the tools are of great delicacy, requiring accurate adjustment every time they are used; and in many cases, the time employed in adjusting bears a large proportion to that employed in using the tool. The sliding-rest, the dividing and the drilling engine are of this kind: and hence, in manufactories of sufficient extent, it is found to be good economy to keep one machine constantly employed in one kind of work: one lathe, for example, having a screw motion to its sliding-rest along the whole length of its bed, is kept constantly making cylinders; another, having a motion for equalizing the velocity of the work at the point at which it passes the tool, is kept for facing surfaces; whilst a third is constantly employed in cutting wheels."
I am very far from implying that these different considerations are of no weight; but I think there are counter-considerations which are overlooked. If one kind of muscular or mental labour is different from another, for that very reason it is to some extent a rest from that other; and if the greatest vigour is not at once obtained in the second occupation, neither could the first have been indefinitely prolonged without some relaxation of energy. It is a matter of common experience that a change of occupation will often afford relief where complete repose would otherwise be necessary, and that a person can work many more hours without fatigue at a succession of occupations, than if confined during the whole time to one. Different occupations
