To be efficient, exercise must be steady and regular. A long tramp or a violent burst of tennis or some other game once a week can never make up for days of comparative inaction. More than that, it is almost dangerous. The waste products have meanwhile accumulated to such an extent that, if they are suddenly stirred up, they are apt to produce a severe headache or a bilious attack. People are often puzzled and discouraged when after a long walk on a Saturday afternoon they feel so done up as to be unfit for anything for the next twenty hours. The exercise should be daily, so that the waste matter is eliminated regularly. To realise the difference between this method and the one we have just been discussing, notice the effect of dusting a room once a week compared with doing it each day. It is the difference between an almost imperceptible quantity of dust and a cloud that fills the room and threatens to choke you. [Sidenote: Violent exercise and cramp.] There is one result of irregular exercise that needs to be referred to here. It is cramp. This painful complaint is due to a deposit of waste matter in some muscle, which it causes to contract violently owing to the local irritation set up. It is liable to occur after hard or prolonged exercise of any sort, especially in those who only take it now and then. Sometimes, however, it is found in those who never take part in games, as in elderly people, for instance. In this case it is often due to the habit of walking beyond the ordinary pace. There are some temperaments which never allow their possessors to walk quietly, even though age or some infirmity demands it, and such persons are very liable to wake up at night suffering from cramp. The treatment is to avoid too violent exercise or to walk in a more leisurely fashion, as the case may be. When it has come on, the remedy is to rub the affected part vigorously, or put it into hot water if such is available. [Sidenote: Outdoor exercise.]