what is really the case; and the student has made his first effort towards the acquisition of sound knowledge, when he has learnt to familiarize himself with the idea that the earth, after all, may be nothing but a great star.

How correct such an idea may be, and with what limitations and modifications it is to be admitted, we shall see presently.

(14.) It is evident, that, to form any just notions of the arrangement, in space, of a number of objects which we cannot approach and examine, but of which all the information we can gain is by sitting still and watching their evolutions, it must be very important for us to know, in the first instance, whether what we call sitting still is really such: whether the station from which we view them, with ourselves, and all objects which immediately surround us, be not itself in motion, unperceived by us; and if so, of what nature that motion is. The apparent places of a number of objects, and their apparent arrangement with respect to each other, will of course be materially dependent on the situation of the spectator among them; and if this situation be liable to change, unknown to the spectator himself, an appearance of change in the respective situations of the objects will arise, without the reality. If, then, such be actually the case, it will follow that all the movements we think we perceive among the stars will not be real movements, but that some part, at least, of whatever changes of relative place we perceive among them must be merely apparent, the results of the shifting of our own point of view; and that, if we would ever arrive at a knowledge of their real motions, it can only be by first investigating our own, and making due allowance for its effects. Thus, the question whether the earth is in motion or at rest, and if in motion, what that motion is, is no idle inquiry, but one on which depends our only chance of arriving at true conclusions respecting the constitution of the universe.

(15.) Nor let it be thought strange that we should speak of a motion existing in the earth, unperceived by its inhabitants: we must remember that it is of the earth as a whole, with all that it holds within its substance, or sustains

on its surface, that we are speaking; of a motion common to the solid mass beneath, to the ocean which flows around it, the air that rests upon it, and the clouds which float above it in the air. Such a motion, which should displace no terrestrial object from its relative situation among others, interfere with no natural processes, and produce no sensations of shocks or jerks, might, it is very evident, subsist undetected by us. There is no peculiar sensation which advertises us that we are in motion. We perceive jerks, or shocks, it is true, because these are sudden changes of motion, produced, as the laws of mechanics teach us, by sudden and powerful forces acting during short times; and these forces, applied to our bodies, are what we feel. When, for example, we are carried along in a carriage with the blinds down, or with our eyes closed (to keep us from seeing external objects), we perceive a tremor arising from inequalities in the road, over which the carriage is successively lifted and let fall, but we have no sense of progress. As the road is smoother, our sense of motion is diminished, though our rate of travelling is accelerated. Railway travelling, especially by night or in a tunnel, has familiarized every one with this remark. Those who have made aeronautic voyages testify that with closed eyes, and under the influence of a steady breeze communicating no oscillatory or revolving motion to the car, the sensation is that of perfect rest, however rapid the transfer from place to place.

(16.) But it is on shipboard, where a great system is maintained in motion, and where we are surrounded with a multitude of objects which participate with ourselves and each other in the common progress of the whole mass, that we feel most satisfactorily the identity of sensation between a state of motion and one of rest. In the cabin of a large and heavy vessel, going smoothly before the wind in still water, or drawn along a canal, not the smallest indication acquaints us with the way it is making. We read, sit, walk, and perform every customary action as if we were on land. If we throw a ball into the air, it falls back into our hand; or if we drop it, it alights at our feet. Insects buzz around us as in the

free air; and smoke ascends in the same manner as it would do in an apartment on shore. If, indeed, we come on deck, the case is, in some respects, different; the air, not being carried along with us, drifts away smoke and other light bodies-such as feathers abandoned to it - apparently, in the opposite direction to that of the ship's progress; but, in reality, they remain at rest, and we leave them behind in the air. Still, the illusion, so far as massive objects and our own movements are concerned, remains complete; and when we look at the shore, we then perceive the effect of our own motion transferred, in a contrary direction, to external objects — external, that is, to the system of which we form a part.

"Provehimur portu, terræque urbesque recedunt."

(17.) In order, however, to conceive the earth as in motion, we must form to ourselves a conception of its shape and size. Now, an object cannot have shape and size unless it is limited on all sides by some definite outline, so as to admit of our imagining it, at least, disconnected from other bodies, and existing insulated in space. The first rude notion we form of the earth is that of a flat surface, of indefinite extent in all directions from the spot where we stand, above which are the air and sky; below, to an indefinite profundity, solid matter. This is a prejudice to be got rid of, like that of the earth's immobility;- but it is one much easier to rid ourselves of, inasmuch as it originates only in our own mental inactivity, in not questioning ourselves where we will place a limit to a thing we have been accustomed from infancy to regard as immensely large; and does not, like that, originate in the testimony of our senses unduly interpreted. On the contrary, the direct testimony of our senses lies the other way. When we see the sun set in the evening in the west, and rise again in the east, as we cannot doubt that it is the same sun we see after a temporary absence, we must do violence to all our notions of solid matter, to suppose it to have made its way through the substance of the earth. It must, therefore, have gone under it, and that not by a mere subterraneous channel; for if we notice the points where it

sets and rises for many successive days, or for a whole year, we shall find them constantly shifting, round a very large extent of the horizon; and, besides, the moon and stars also set and rise again in all points of the visible horizon. The conclusion is plain: the earth cannot extend indefinitely in depth downwards, nor indefinitely in surface laterally; it must have not only bounds in a horizontal direction, but also an under side round which the sun, moon, and stars can pass; and that side must, at least, be so far like what we see, that it must have a sky and sunshine, and a day when it is night to us, and vice versâ; where, in short,

(18.) As soon as we have familiarized ourselves with the conception of an earth without foundations or fixed supports-existing insulated in space from contact of every thing external, it becomes easy to imagine it in motion-or, rather, difficult to imagine it otherwise; for, since there is nothing to retain it in one place, should any causes of motion exist, or any forces act upon it, it must obey their impulse. Let us next see what obvious circumstances there are to help us to a knowledge of the shape of the earth.

(19.) Let us first examine what we can actually see of its shape. Now, it is not on land (unless, indeed, on uncommonly level and extensive plains), that we can see any thing of the general figure of the earth;—the hills, trees, and other objects which roughen its surface, and break and elevate the line of the horizon, though obviously bearing a most minute proportion to the whole earth, are yet too considerable with respect to ourselves and to that small portion of it which we can see at a single view, to allow of our forming any judgment of the form of the whole, from that of a part so disfigured. But with the surface of the sea or any vastly extended level plain, the case is otherwise. If we sail out of sight of land, whether we stand on the deck of the ship or climb the mast, we see the surface of the sea- not losing

itself in distance and mist, but terminated by a sharp, clear, well-defined line or offing as it is called, which runs all round us in a circle, having our station for its centre. That this line is really a circle, we conclude, first, from the perfect apparent similarity of all its parts; and, secondly, from the fact of all its parts appearing at the same distance from us, and that, evidently, a moderate one; and thirdly, from this, that its apparent diameter, measured with an instrument called the dip sector, is the same (except under some singular atmospheric circumstances, which produce a temporary distortion of the outline), in whatever direction the measure is taken,-properties which belong only to the circle among geometrical figures. If we ascend a high eminence on a plain (for instance, one of the Egyptian pyramids), the same holds good.

(20.) Masts of ships, however, and the edifices erected by man, are trifling eminences compared to what nature itself affords; Etna, Teneriffe, Mowna Roa, are eminences from which no contemptible aliquot part of the whole earth's surface can be seen; but from these again-in those few and rare occasions when the transparency of the air will permit the real boundary of the horizon, the true sea-line, to be seenthe very same appearances are witnessed, but with this remarkable addition, viz. that the angular diameter of the visible area, as measured by the dip sector, is materially less than at a lower level; or, in other words, that the apparent size of the earth has sensibly diminished as we have receded from its surface, while yet the absolute quantity of it seen at once has been increased.

(21.) The same appearances are observed universally, in every part of the earth's surface visited by man. Now, the figure of a body which, however seen, appears always circular, can be no other than a sphere or globe.

(22.) A diagram will elucidate this. Suppose the earth to be represented by the sphere LHN Q, whose centre is C, and let A, G, M be stations at different elevations above various points of its surface, represented by a, g, m respectively. From each of them (as from M) let a line be drawn, as MN n,

C