The exercise he takes should be frequent; but not violent. Motion preserves the firmness of the parts, and elasticity of the vessels; it prevents that aggregation of thick humours which he is most to fear. A sedentary life always produces weakness, and that mischief always follows: weak eyes are gummy, weak lungs are clogged with phlegm, and weak bowels waste themselves in vapid diarrhoeas. Let him invite himself abroad, and let his friends invite him by every innocent inducement. For me, I should advise above all other things the study of nature. Let him begin with plants: he will here find a continual pleasure, and continual change; fertile of a thousand useful things; even of the utility we are seeking here. This will induce him to walk; and every hedge and hillock, every foot-path side, and thicket, will afford him some new object. He will be tempted to be continually in the air, and continually to change the nature and quality of the air, by visiting in succession the high lands and the low, the lawn, the heath, the forest. He will never want inducement to be abroad; and the unceasing variety of the subjects of his observation, will prevent his walking hastily: he will pursue his studies in the air; and that contemplative turn of mind, which in his closet threatened his destruction, will thus become the great means of his recovery. If the mind tire upon this, from the repeated use, another of nature's kingdoms opens itself at once upon him; the plant he is weary of observing, feeds some insect he may examine; nor is there a stone that lies before his foot, but may afford instruction and amusement. Even what the vulgar call the most abject things will shew a wonderful utility; and lead the mind, in pious contemplation higher than the stars. The poorest moss that is trampled under foot, has its important uses: is it at the bottom of a wood we find it? why there it shelters the fallen seeds; hides them from birds, and covers them from frost; and thus becomes the foster father of another forest! creeps it along the surface of a rock? even there its good is infinite! its small roots run into the stone, and the rains make their way after them; the moss having lived its time dies; it rots and with the mouldered fragments of the stone forms earth; wherein, after a few successions, useful plants may grow, and feed more useful cattle![19] Is there a weed more humble in its aspect, more trampled on, or more despised than knot grass! no art can get the better of its growth, no labour can destroy it; 'twere pity if they could, for the thing lives where nothing would of use to us; and its large and most wonderfully abundant seeds, feed in hard winters, half the birds of Heaven.