When I have established a pair of well-pronounced feathering-calluses on my thumbs, when I am in training so that I can do my fifteen miles at a stretch without coming to grief in any way, when I can perform my mile in eight minutes or a little less, then I feel as if I had old Time's head in chancery, and could give it to him at my leisure. I do not deny the attraction of walking. I have bored this ancient city through and through in my daily travels, until I know it as an old inhabitant of a Cheshire knows his cheese. Why, it was I who, in the course of these rambles, discovered that remarkable avenue called _Myrtle Street_, stretching in one long line from east of the Reservoir to a precipitous and rudely paved cliff which looks down on the grim abode of Science, and beyond it to the far hills; a promenade so delicious in its repose, so cheerfully varied with glimpses down the northern slope into busy Cambridge Street with its iron river of the horse-railroad, and wheeled barges gliding back and forward over it,--so delightfully closing at its western extremity in sunny courts and passages where I know peace, and beauty, and virtue, and serene old age must be perpetual tenants,--so alluring to all who desire to take their daily stroll, in the words of Dr. Watts,-- "Alike unknowing and unknown,"-- that nothing but a sense of duty would have prompted me to reveal the secret of its existence. I concede, therefore, that walking is an immeasurably fine invention, of which old age ought constantly to avail itself. Saddle-leather is in some respects even preferable to sole-leather. The principal objection to it is of a financial character. But you may be sure that Bacon and Sydenham did not recommend it for nothing. One's _hepar_, or, in vulgar language, liver,--a ponderous organ, weighing some three or four pounds,--goes up and down like the dasher of a churn in the midst of the other vital arrangements, at every step of a trotting horse. The brains also are shaken up like coppers in a moneybox. Riding is good, for those that are born with a silver-mounted bridle in their hand, and can ride as much and as often as they like, without thinking all the time they hear that steady grinding sound as the horse's jaws triturate with calm lateral movement the bank-bills and promises to pay upon which it is notorious that the profligate animal in question feeds day and night. Instead, however, of considering these kinds of exercise in this empirical way, I will devote a brief space to an examination of them in a more scientific form.