Against this objection may be placed the equally serious objection to day-work, that every interruption, when you are particularly anxious not to be interrupted, causes a definite loss and injury to the nervous system. The choice must therefore be made between two dangers, and if they are equally balanced there can be no hesitation, because all the _literary_ interests of an author are on the side of the most tranquil time. Literary work is always sure to be much better done when there is no fear of disturbance than under the apprehension of it; and precisely the same amount of cerebral effort will produce, when the work is uninterrupted, not only better writing, but a much greater quantity of writing. The knowledge that he is working well and productively is an element of health to every workman because it encourages cheerful habits of mind. In the division of time it is an excellent rule for adults to keep it as much as possible _in large masses_, not giving a quarter of an hour to one occupation and a quarter to another, but giving three, four, or five hours to one thing at a time. In the case of children an opposite practice should be followed; they are able to change their attention from one subject to another much more easily than we can, whilst at the same time they cannot fix their minds for very long without cerebral fatigue leading to temporary incapacity. The custom prevalent in schools, of making the boys learn several different things in the course of the day, is therefore founded upon the necessities of the boy-nature, though most grown men would find that changes so frequent would, for them, have all the inconveniences of interruption. To boys they come as relief, to men as interruption. The reason is that the physical condition of the brain is different in the two cases; but in our loose way of talking about these things we may say that the boy's ideas are superficial, like the plates and dishes on the surface of a dinner-table, which may be rapidly changed without inconvenience, whereas the man's ideas, having all struck root down to the very depths of his nature, are more like the plants in a garden, which cannot be removed without a temporary loss both of vigor and of beauty, and the loss cannot be instantaneously repaired. For a man to do his work thoroughly well, it is necessary that he should dwell in it long enough at a time to get all the powers of his mind fully under command with reference to the particular work in hand, and he cannot do this without tuning his whole mind to the given diapason, as a tuner tunes a piano. Some men can tune their minds more rapidly, as violins are tuned, and this faculty may to a certain extent be acquired by efforts of the will very frequently repeated. Cuvier had this faculty in the most eminent degree. One of his biographers says: "His extreme facility for study, and of directing all the powers of his mind to diverse occupations of