5. _Every part of the body ought to be exercised._ This is done spontaneously by the infant. Every muscle of his body acts in sheer delight. The evils of exercise confined to particular groups of muscles have been already described. Lawn tennis is very valuable as affording exercise for both limb and trunk muscles. 6. _Exercise should not be taken immediately after meals_, as thus digestion is interfered with. 7. _Exercise should be taken, as far as possible, in the open air._ A small amount of exercise out of doors is much more invigorating than a large amount indoors. =The Forms of Exercise= taken may be divided into recreative and _educational_, though both of course may be recreative under many circumstances. The primarily recreative exercises, such as rowing, cricket, football, tennis, hockey, will, it may be hoped, be never replaced by educational gymnastics, though the latter possess a high value. The recreative influence as well as the influence on the power of self-control of such games as cricket and football render them of national importance. Educational gymnastics can be applied to exercise the muscles of any part of the body, and can be exactly graduated to individual requirements. Singing, speaking, and reading aloud, are forms of muscular exercise very much neglected, and they are particularly important, as the lungs and voice are by these means greatly strengthened, and rendered much less liable to the inroads of disease. Professor Haughton has shown that the work done by a man walking on a level surface at the rate of three miles an hour is equivalent to raising his own weight, _plus_ the weight he carries through 1∕20 of the distance walked. Thus, if W = weight of the man, W^1 = weight carried by him, D = distance walked in feet, C = co-efficient of traction (1∕20, at three miles an hour), then we obtain by the following formula the amount of work done, the co-efficient of traction being multiplied by 2,240 (the number of pounds in a ton) to obtain the result in foot-tons. (W + W^1) × D/(C × 2,240) In ascending a height, a man raises his whole weight through the height ascended. _A regiment of soldiers marches ten miles, each carrying a weight of 60 lbs. What amount of work is performed by each soldier?_ If we assume the average weight of each soldier to be 150 lbs., and that the march was at the rate of three miles an hour, then— (150 + 60) × 10 × 5,280/(20 × 2,240) = 247·5 foot-tons. In this example it is assumed that the march is on entirely level ground that all weights are carried in the most convenient manner, and that the rate of travel is three miles an hour. Velocity is gained at the expense of carrying power. It has been found that the amount of work is generally inversely as the square of the velocity. Haughton has determined from Weber’s calculations the co-efficient of resistance for three velocities.