"Up

absolute direction considered independently. wards," in and for whatever locality it is used, is conventionally understood to imply retreat from the Earth, and "downwards" approach to it. Since these meanings are respectively attributed to the terms in question for all spots on the surface of the world, and throughout all the various attitudes which the globe assumes in the course of its orbit, they can plainly have no more than the restricted signification which has been mentioned. For if their import was not taken to be thus limited, it would comprise changes of place in all directions which are conceivable, and of which half the number would severally be diametrically opposite to the other half. Lines of possible movement are, as need scarcely be observed, absolutely infinite in number,

so that the expressions which have been criticised would, if not interpreted in the narrow sense which has been

pointed out, be wholly devoid of any real meaning whatever.

For reasons which have now become obvious the saucers of a pair of scales do not hang with their centres in a perpendicular line below their respective points of suspension if the pillar which supports the beam be regarded as vertical. That is to say, lines drawn from those centres to the beam-ends above them are not parallel to one another, or to the scale post, but as they approach the ground they make some slight convergence. This deviation from parallelism, although it is so infinitesimally small as to elude any observations which our senses can bring to bear on it, and defies any tests by which we might endeavour to appraise it, has nevertheless a true existence. Lines drawn as supposed would meet at the centre of the earth, and so of course would the saucers if they could swing so far.

It has been stated that the centre of the globe is the seat of the whole attraction of gravity which the entire mass of our world exercises. The thorough assimilation, in medical phraseology, of this fact by the mental digestion may be promoted if it be administered. in another shape in which it will be readily taken up by the intellectual absorbents. The attraction of gravity is found by experiment to be governed by a particular law to which we referred just now, and to which it conforms, under all circumstances whatever, with strict and implicit obedience. This Ordinance of Nature, when expressed, for simplicity's sake, in the most general terms possible, is as follows, namely, that the attraction of the gravity of any given mass wanes

according to a certain well-ascertained scale of degrees which is in precise correspondence with the length of the distance through which the force acts. Now if the surface and not the centre of the Earth were to be taken as the source whence attraction emanates, results inconsistent with the law in question would be given by practical experiments founded on such an assumption as this, showing that an erroneous basis of calculation had been laid.

We have seen that the centre of gravity of any, and every, body is that point about which, if it be supported, the whole mass will balance, and therefore rest quiet and contented. We have also seen that if this point be not supported, it will descend to as low a position as it can reach, travelling along the straight road leading from its own position for the time being towards the centre of the earth. Let us take a couple of instances to show how this occurs in actual practice, and with what results.

AP

Here we have a three-cornered board of the shape of a pyramid. Since it is of a regular figure, and of one uniform substance, its centre of gravity (leaving out of account its thickness which is so small as to be immaterial) will be at the middle of its surface. Set

this pyramid upright upon its base-thus-at the edge of the table. Suspend a string, with a weight at its lower end, from the centre of gravity of the pyramid in such a manner that it may hang quite free. The weight is bound to get as near to the centre of gravity of the earth as it can, and the string by which it hangs must thus represent part of a straight line which, if it were to be continued sufficiently far, would join the centre of gravity of the earth with that of the pyramid.

The pyramid remains at rest. Why? Because its centre of gravity is supported by what may be regarded as a vertical prop, constituted by a line of particles which are a portion of the substance of the pyramid, and which, as it were, form a pillar whose base rests upon the table. If you tilt the pyramid a little up on one side, you lift, as you see, the lower end of the prop from the table. The pyramid, therefore, will not continue at rest after your hand is withdrawn, since its centre of gravity, having been deprived of the support which it gained from the table through the prop, will now move downwards, till the original conditions of stability have been restored. The pyramid may be considerably tilted, yet it will still continue to resume its first position as soon as permitted to do so, provided only that it is not made to bear over to one side to such a degree as that the line dropped perpendicularly from its centre of gravity falls beyond the extremity of the base in that same quarter. When that occurs, the attraction of gravity combines at once with the lateral inclination coming from the tilt, instead of counteracting it, and helps to bring the pyramid down by pulling it round over the terminal point of its base, turning it, as it were, upon a pivot.

A pyramid, therefore, when standing on its base, has such a figure and "pose." as are calculated to impart to it a resolute conservatism of attitude, since the effect of any disturbance of its equilibrium, provided that this does not exceed a certain definite limit, is to excite an opposing and restitutive action on the part of gravity. A body so circumstanced in respect of its balance is said to be in a position of " stable equilibrium."

It is quite unnecessary to advert to the obvious reasons of a similar, or, as it might, perhaps, be more accurate to say, converse kind, why a pyramid which poises on one of its tips has no stability of attitude, but topples over on its side on the slightest provocation. When it is thus insecurely balanced, with a disposition and destiny, if it rocks at all, to fall irretrievably, it is said to be in a position of "unstable equilibrium."

Our examination of the conditions on which the stability of a pyramid depends has incidentally rendered us able to discern at a glance why an overloaded waggon will upset on a bit of road slanting sharply towards the ditch on either side, which it would traverse without risk or disaster if it were empty. And there can be noone who has gone along with what has been already said who must not fully understand why an egg is contented enough to lie lazily on its side, but cannot be made to stand in what may be called an upright attitude. You recollect the old story of Columbus and the egg. Some bets were made at a party at which Columbus was present that he could not make an egg keep erect on either of its extremities. He cut