We need this faith in exercise--this firm conviction of its necessity--the sort of conviction that makes a man go out in all weathers, and leave the most urgent intellectual labor for the mere discipline and hardening of the body. Few students possess this faith in its purity. It is hard to believe that we shall get any good from exercise proportionate to the sacrifice of time. The incompatibility between the physical and the intellectual lives is often very marked if you look at small spaces of time only; but if you consider broader spaces, such as a lifetime, then the incompatibility is not so marked, and gives place to a manifest conciliation. The brain is clearer in vigorous health than it can be in the gloom and misery of sickness; and although health may last for a while without renewal from exercise, so that if you are working under pressure for a month the time given to exercise is so much deducted from the result, it is not so for the life's performance. Health sustained for many years is so useful to the realization of all considerable intellectual undertakings, that the sacrifice to the bodily well-being is the best of all possible investments. Franklin's theory about concentrating his exercise for the economy of time was founded upon a mistake. Violent exertion for minutes is _not_ equivalent to moderate exercise for hours. The desire to concentrate good of various kinds into the smallest possible space is one of the commonest of _human_ wishes, but it is not encouraged by the broader economy of nature. In the exercise of the mind every teacher is well aware that time is an essential factor. It is necessary to _live_ with a study for hundreds or thousands of hours before the mind can assimilate as much of the subject as it may need; and so it is necessary to live in exercise during a thousand hours of every year to make sure of the physical benefits. Even the fresh air itself requires time to renovate our blood. The fresh air cannot be concentrated; and to breathe the prodigious quantities of it which are needed for perfect energy, we must be out in it frequently and long.