necessary to compress them evidently shews,--into a tendency from each other; _both_ tendencies, indeed, being _inexplicable_, but the one in no respect more so than the other. For this apparent digression, on a point of general physics, I make no apology, as it is absolutely necessary for illustrating the particular case to which I am to proceed. The consideration of it requires, what the whole of this discussion, indeed, has already required from you, no small exercise of patient attention; but I trust that I sufficiently prepared you for this, in a former Lecture, when I stated the importance of such attention, not merely in relation to the subject considered at the time, but as a part of your mental discipline, and the advantage which might thus be derived to your intellectual character, from the very difficulties which the subject presents. It is in philosophy, as in many a fairy tale. The different obstacles which the hero encounters, are not progressively greater and greater; but his most difficult achievements are often at the very commencement of his career. He begins, perhaps, with attacking the castle of some enchanter, and has to force his way, unassisted, through the griffins and dragons that oppose his entrance. He finishes the adventure, with the death of the magician--and strips him of some ring, or other talisman, which renders his subsequent adventures comparatively easy and secure. I cannot venture to say, indeed, that a perfect acquaintance with the difficulties of the present question, and of some of the late questions which have engaged us, will be such a talisman to you, in your future career of intellectual science. But I may safely say, that the habit of attentive thought, which the consideration of subjects, so abstract, necessarily produces, in those who are not too indolent to give attention to them, or too indifferent to feel interest in them, is more truly valuable than any talisman, of which accident or force might deprive you. The _magic_ with which this endows you, is not attached to a _ring_, or a gem, or any thing external; it lives, and lives forever, in the very essence of your minds. When a billiard ball, on being struck, approaches another, which is at rest, it soon arrives at the point of seeming, but not actual contact, at which their mutual attraction ceases, and the force which it has acquired still carrying it on, it passes this bounding point, and arrives at a point at which repulsion has already begun. Accordingly the body, formerly at rest, now flies off, on a principle precisely similar, (though the mere direction be opposite) to that by which the same ball, if dropped from a hand that supported it, would, without the actual impulse of any body, have quitted its state of rest, as in the present case, and have gravitated, or, which is the same thing, have moved of itself toward the earth.