[Illustration: Fig. 43.--Student exercising in the School Gymnasium on the
Rowing Machine. (From a photograph.)]

The proper amount of exercise must vary greatly with circumstances. It may
be laid down as a fairly safe rule, that a person of average height and
weight, engaged in study or in any indoor or sedentary occupation, should
take an amount of exercise equivalent to walking five or six miles a day.
Growing children, as a rule, take more exercise than this, while most men
working indoors take far less, and many women take less exercise than men.
Exercise may be varied in many ways, the more the better; but for the most
part it should always be taken in the open air.

89. Time for Exercise. It is not prudent to do hard work or take
severe exercise, just before or just after a full meal. The best time is
one or two hours after a meal. Vigorous exercise while the stomach is
busily digesting food, may prove injurious, and is apt to result sooner or
later in dyspepsia. On the other hand, severe exercise should not be taken
on an empty stomach. Those who do much work or study before breakfast,
should first take a light lunch, just enough to prevent any faint feeling.
With this precaution, there is no better time for moderate exercise than
the early morning.

In the case of children, physical exercises should not be undertaken when
they are overtired or hungry. Neither is it judicious for adults to take
vigorous exercise in the evening, after a long and arduous day's work.

90. Walking, Running, and Jumping. Walking is generally regarded as
the simplest and most convenient mode of taking exercise. Man is
essentially a walking animal. When taken with a special object in view, it