Outdoor exercises, such as walking, running, swimming, and hill and mountain climbing, possess the very great advantage that indulging in them demands no expense, and are, therefore, open to all. All these exercises develop the lungs and chest and strengthen the heart, and are, therefore, classed under exercises of endurance. =The Effect of Walking on the Heart and Lungs.=—In a slow walk the respiratory action is almost double the ordinary amount; in walking at an ordinary pace, that is, about three miles an hour, it is four times as great; while in a vigorous or hasty walk, that is, at a rate of from four to six miles an hour, it is from seven to nine times as great. The change of speed from three to five miles an hour elevates an automatic, listless occupation into a vigorous exercise, employing many new muscle groups and stimulating the heart, lungs, and skin, while the change from a smooth, level road to the broken ground of the mountain-side may be dangerous for many a one who is able to walk at a moderate speed on level ground. In walking the clothing must be sufficiently loose not to interfere with the more rapid respirations and the increased action of the heart. When the heart cannot keep pace with the demanded speed of the circulation, a “stitch” ensues, and getting one’s second wind means that the heart has succeeded in accommodating itself to the strain. Too great a “stitch,” resulting in absolute breathlessness, is a warning that must not be disregarded. This increase in the respiratory action is important, as compared with the brief and transient increase from exercise with apparatus, because a quick walk can easily be kept up for several hours. The fatigue is small, because, in the first place, of the abundant supply of oxygen; the will is scarcely used, and walking is almost automatic, partly because the muscle areas used are large, and each movement prepares for the next. Walking is a heart and lung exercise of a very excellent sort. _The Effect of Walking on the Movements of the Blood in the Legs._—The circulation of the blood depends on the pumping of the heart, which is in turn affected by the suction action of the lungs and the muscular movements of contraction and relaxation which go on rhythmically. While in prolonged standing the veins of the legs become fuller, and the circulation of blood in them more sluggish, and by the laws of gravitation the blood is kept down in them. Hence also in walking slowly with short steps the legs remain overfull of blood and become heavy. Instead of being quickened, the circulation is actually hindered, for the waste-products are not carried away quickly enough. Hence, slow walking soon causes a feeling of fatigue, while the vigorous walker, going along with long strides, keeps fresh.