ance of the excepted case, immediately prior to its occurrence?'

As Babbage further showed, a calculating engine, after proceeding through any required number of motions according to a first law, may be made suddenly to suffer a change, so that it shall then commence to calculate according to a wholly new law. After giving the natural numbers for any finite time, it might suddenly begin to give triangular, or square, or cube numbers, and these changes might theoretically be conceived as occurring time after time. Now if such occurrences can be designed and foreseen by a human artist, it is surely within the capacity of the Divine Artist to provide for similar changes of law in the mechanism of the atom, or the construction of the heavens.

Physical science, so far as its highest speculations can be trusted, gives some indication of a change of law in the past history of the Universe. According to Sir W. Thomson's deductions from Fourier's Theory of Heat, we can trace down the dissipation of heat by conduction and radiation to an infinitely distant time when all things will be uniformly cold. But we cannot similarly trace the heat-history of the Universe to an infinite distance in the past. For a certain negative value of the time the formulæ give impossible values, indicating that there was some initial distribution of heat which could not have resulted, according to known laws of nature, from any previous distributions. There are other cases in which a consideration of the dissipation of energy leads to the conception of a limit to the antiquity of the present order of things. Human science, of course, is fallible, and

Tait's Thermodynamics,' p. 38. Cambridge Mathematical Journal,' vol. iii. p. 174.

h Clerk Maxwell's Theory of Heat,' p. 245.

some oversight or erroneous simplification in these theoretical calculations may afterwards be discovered; but as the present state of scientific knowledge is the only ground on which erroneous interpretations of the uniformity of nature and the reign of law are founded, I am right in appealing to the present state of science in opposition to these interpretations. Now the theory of heat places us in the dilemma either of believing in Creation at an assignable date in the past, or else of supposing that some inexplicable change in the working of natural laws then took place. Physical science gives no countenance to the notion of infinite duration of matter in one continuous course of existence. And if in time past there has been a discontinuity of law, why may there not be a similar event awaiting the world in the future. Infinite ingenuity could have implanted some agency in matter so that it might never yet have made its tremendous powers manifest. We have a very good theory of the conservation of energy, but the foremost physicists do not deny that there may possibly be forms of energy, neither kinetic nor potential, and therefore of unknown nature.

We can imagine reasoning creatures dwelling in a world where the atmosphere was a mixture of oxygen and inflammable gas like the fire-damp of coal mines. If devoid of fire, they might have lived on through long ages in complete unconsciousness of the tremendous forces which a single spark could call into play. In the twinkling of an eye new laws might have come into action, and the poor reasoning creatures who were so confident in their knowledge of the uniform conditions of their world, might have had no time even to speculate upon the overthrow of all their theories. Can we with our finite knowledge be sure that such an overthrow of our theories is impossible?

i Maxwell's 'Theory of Heat,' p. 92.

The Ambiguous Expression.-Uniformity of Nature.

I have asserted that a serious misconception arises from an ambiguous interpretation of the expression Uniformity of Nature. Every law of nature is the statement of a certain uniformity observed to exist among phenomena, and since the laws of nature are supposed to be invariably obeyed it seems to follow that the course of nature itself is uniform, so that we can safely judge of the future by the present. This inference is supported by some of the most profound results of physical astronomy. Laplace proved that the planetary system was stable, so that no one of the perturbations which planet produces upon planet shall become so great as to cause a disruption, and a permanent alteration in the planetary orbits. A full comprehension of the law of gravity shows that all such disturbances are essentially periodic, so that after the lapse of millions of years the planets will all return to the same relative positions and a new cycle of disturbances will

commence.

As other branches of inquiry progress, we seem to gain assurance that no great alteration of the world's condition is to be expected. A conflict with a comet has long been a cause of fear to some persons, but now it is credibly asserted that we have passed through a comet's tail without the fact being known at the time, or manifested by any more serious a phenomenon than a slight luminosity of the heavens. More recently still the earth is said to have actually touched the comet Biela, and the only result was a beautiful and perfectly harmless display of radiating meteors. A decrease in the heating power of the sun seems to be the next most probable circumstance from which we might fear an extinction of life on the earth. But calculations founded on reasonable physical

data show that no appreciable change can be going on, and experimental data to indicate any change are wholly wanting. Geological investigations show indeed that there have been extensive variations of climate in past times; vast glaciers and icebergs have swept over the temperate regions at one time, and tropical vegetation has flourished. near the poles at another time. But here again the vicissitudes of climate assume a periodic character, so that the ultimate stability of the earth's condition does not seem to be affected.

All these statements may be reasonable, but they do not in the least establish the Uniformity of Nature in the sense that extensive alterations or sudden catastrophes are impossible. In the first place Laplace's theory of the stability of the planetary system is of an abstract character, as paying regard to nothing but the mutual gravitation of the planetary bodies and the sun. It overlooks several physical causes of change and decay in the system which were not so well known in his day as at present, and it also presupposes the absence of any interruption of the course of things by conflict with foreign astronomical bodies.

It is now commonly acknowledged by astronomers that there are at least two ways in which the vis viva of the planets and satellites may suffer loss. The friction of the tides upon the earth produces a small amount of heat which is radiated into space, and this loss of energy must result in a decrease of the rotational velocity, so that ultimately the terrestrial day will become identical with the year, just as the periods of revolution of the moon upon its own axis and around the earth have already become equal. Secondly, there can now be little doubt that various manifestations of electricity upon the earth's surface depend upon the relative motions of the planets and the sun, which give rise to various periods of increased intensity. Such electrical phenomena must result

in the production and dissipation of heat, the energy of which must be drawn, partially at least, from that of the moving bodies. This effect is probably identical, as I have suggested (vol. ii. p. 213), with the very evident loss of energy of comets attributed to a so-called resisting medium. But whatever be the theoretical explanation of these phenomena, it is almost certain that there exists a tendency to the dissipation of the energy of the planetary system, which will in the indefinite course of time result in the fall of the planets into the sun.

It is hardly probable, however, that the planetary system will be left undisturbed throughout the enormous period of time required for the dissipation of its energy in this way. Conflict with other bodies is so far from being improbable, that it becomes approximately certain when we take very long intervals of time into account. As regards cometary conflicts, I am by no means satisfied with the negative conclusions drawn from the remarkable display on the evening of the 27th of November, 1872. We may often have passed through the tails of comets, which are probably electrical manifestations no more substantial than the aurora borealis. Every remarkable shower of shooting stars may also be considered as proceeding from a cometary body, so that we may be said to have passed through the thinner parts of various comets. But the earth has probably never passed, in times of which we have any record, through the nucleus of a comet, which consists perhaps of a dense swarm of small meteorites. We can only speculate upon the effects which might be produced by such a conflict, but it would probably be a much more serious event than any yet registered in history. The probability of its occurrence, too, can hardly be assigned; for though the probability of conflict with any one cometary nucleus is almost infinitesimal, yet the number of comets is immensely great (vol. ii. p. 11).