The man who walks twenty miles with a motive that engrosses his soul does not attend to his slight fatigue of body when he comes in; but double his motive, and set him to walk another twenty miles, quadruple it, and let him start a third time, and so on; and the length of his walk will ultimately depend upon muscle and not mind.  Powell, for a motive of ten guineas, would have walked further probably than Mr Godwin, for a motive of half a million.  A motive of uncommon power acting upon a frame of moderate strength would, perhaps, make the man kill himself by his exertions, but it would not make him walk a hundred miles in twenty-four hours.  This statement of the case shews the fallacy of supposing that the person was really not at all tired in his first walk of twenty miles, because he did not appear to be so, or, perhaps, scarcely felt any fatigue himself.  The mind cannot fix its attention strongly on more than one object at once.  The twenty thousand pounds so engrossed his thoughts that he did not attend to any slight soreness of foot, or stiffness of limb.  But had he been really as fresh and as alert, as when he first set off, he would be able to go the second twenty miles with as much ease as the first, and so on, the third, &c.  Which leads to a palpable absurdity.  When a horse of spirit is nearly half tired, by the stimulus of the spur, added to the proper management of the bit, he may be put so much upon his mettle, that he would appear to a standerby, as fresh and as high spirited as if he had not gone a mile.  Nay, probably, the horse himself, while in the heat and passion occasioned by this stimulus, would not feel any fatigue; but it would be strangely contrary to all reason and experience, to argue from such an appearance that, if the stimulus were continued, the horse would never be tired.  The cry of a pack of hounds will make some horses, after a journey of forty miles on the road, appear as fresh, and as lively, as when they first set out.  Were they then to be hunted, no perceptible abatement would at first be felt by their riders in their strength and spirits, but towards the end of a hard day, the previous fatigue would have its full weight and effect, and make them tire sooner.  When I have taken a long walk with my gun, and met with no success, I have frequently returned home feeling a considerable degree of uncomfortableness from fatigue.  Another day, perhaps, going over nearly the same extent of ground with a good deal of sport, I have come home fresh, and alert.  The difference in the sensation of fatigue upon coming in, on the different days, may have been very striking, but on the following mornings I have found no such difference.  I have not perceived that I was less stiff in my limbs, or less footsore, on the morning after the day of the sport, than on the other morning.