A senior wrangler has been known to find five or six hours a day of real work at mathematics as much as he could stand.

Of course, work involving little hard physical exertion and hardly any mental effort can go on much longer, but the very monotony which in some ways makes it easy, has a deadening effect.

A factory operative minding a "mule" being asked: "Is it not very hard work always watching and piecing threads ?" answered, "No, but it is very dree work." But the evil effects of too long hours are not confined to the fact that unrest or disputes arise from the state of feeling produced nor to the diminution of production due to fatigue.

Recurrent strains continued over a long period indeed deteriorate even things which are inanimate.

The "fatigue of metals" has been the subject of careful investigations.