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
study, from one quarter of an hour to another, was one of the most
extraordinary qualities of his mind." The Duke of Wellington also
cultivated the habit (inestimably valuable to a public man) of directing
the whole of his attention to the subject under consideration, however
frequently that subject might happen to be changed. But