The amount of exercise which is necessary for health varies with the individual and with age, season, etc., so that it is difficult to state any general rule upon this subject; but if stated in terms of muscular force only, the estimate of Dr. Parkes seems a fair approximation--viz. that every healthy man ought to take daily an amount of exercise equivalent to 150 tons lifted 1 foot, or a walk of about nine miles. The majority of trades and bodily occupations demand at least this amount of work, but in some of them the greater part of the exertion is made only by certain groups of muscles, and they are carried on in crowded and {199} ill-ventilated shops. Such workmen, as well as all who are engaged in sedentary pursuits, require exercise in the open air--exercise which will bring into play the unused muscles and will break the train of thought of the professional man. One of the most important questions with regard to physical exercise is the extent to, and manner in, which it should be provided for in a proper system of education. One of the latest and most instructive articles on this subject is that by Du Bois Reymond in the "Physiology of Exercise," a translation of which is given in the _Popular Science Monthly_ for July and August, 1882. He divides the physical training which is more and more becoming a part of modern systematic education into three classes: The first, the turning, or gymnastics of the Germans; the second, the Swedish system, in which the exercises are limited to very simple though varied movements; and the English system, or rather want of system, consisting largely of athletic games and contests of various kinds. His objection to the Swedish system is that, while it strengthens the muscles, it does not increase the power over composite movements; in other words, it does not exercise the nervous system. Naturally, he prefers the German system to any other, although admitting that the English meets better the demands arising from our structure. "Were the end masterhood in running, jumping, climbing, in dancing, fencing, riding, in swimming, rowing, or skating, then nothing could be more advisable than to practise equally the necessary concatenations in the actions of the ganglion cells, without pausing at the not practically applicable preliminary and intermediate steps of the German turning."