These are some familiar Instances of the Sense we have, after what manner we come to acquire more Strength upon the Use of Exercise, and which every Man almost may recollect, that he has experienc’d more or less in himself; tho’ in other Cases the solid Parts are always strengthening by Exercise, without so plain a Sense of it, as in the Instances above mention’d; as we see what excessive Strength some Men gradually acquire by a constant Practice of vehement Motions, begun when they are young, which growing upon ’em by degrees, they are not so sensible of the Encrease of it. This is the Case of Tumblers, Rope-dancers, and the like, in whom the Nervous and Solid Parts must be incomparably more wound up, more tense than in other People; and thus we see the strongest Men are often thin and Raw-bon’d, as we call it; that is, tho’ daily hard Labour, and great Perspiration carry off a great deal of the grosser Fluids of the Body; yet are the Muscles not Flaccid, but Tense and Firm, capable of greater Actions than the Muscles of those who seem to have a better Habit of Body; which plainly indicates, that Exercise does communicate some Strength to the Nervous Parts, which cannot be any other way procur’d; and that we may argue from the greater to the less; that if healthy Persons may acquire such monstrous Strength by Use, People that are Valetudinary may, by setting themselves upon a resolute and diligent Practice of moderate Exercise, obtain a proportionable Increase of Strength. It may be expected, perhaps, that I should endeavour to explain, how the Fibres come to receive a greater Power to act, by being often put upon Action, and to shew wherein the Elater, the Spring of the Solids, does consist; but this has been attempted so largely by an Eminent Author, Dr. _Baglivi_, and so much to the Disgust of very many, that it’s better to rest contented with plain Experience, than to frame an _Hypothesis_ for the _Modus_ of so abstruse a Method of Nature, which, in all likelyhood, Mankind will ever be ignorant of; but as far as we may reasonably guess, by frequent Distension the Nerves receive a greater quantity of Animal Spirits, because the Limb which is most us’d, grows biggest; and there is reason to induce us to suspect, that the Fibre it self strengthens by Use, has a peculiar Faculty to exert it self more and more, as often as the _Imperium Voluntatis_, the _Fiat of the Will_, sets it upon Motion. But unless we knew the Bond of Union, and understood how the Rational Soul acts upon the Animal Powers, we must be content to be most ignorant, the nearer we approach in our Disquisitions to that Union; but the Experimental Knowledge of these Parts sufficiently reproves those who hope to be deliver’d from some Distempers seated in the Solids, without acting suitably to the Nature of the Solids, the Subject of their Distempers: As for Instance; Suppose a Person, by frequent and unnecessary Use of the Bagnio, and more unnecessary Bleedings; by the