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 about it. But, not wishing to leave anything uninvestigated, he goes round to the other side of the wall. There he finds that one end of the shaft comes through it, and is in mechanical connection with either a steam-engine or a water-wheel; and, by watching what occurs when its motion is checked and renewed, he sees that the engineer shuts off, or turns on, either the steam generated in the boiler of the steam engine, or the descending water whose motion drives the wheel.

I shall not weary the patience of such readers as may have followed me thus far, by tracing out in like detail the further steps of the inquiry, but shall land them in the final conclusion now accepted by every man of science—that the power exerted in both these cases is drawn from solar radiation: the fall of the water which gives motion to the water-wheel being merely the return of that which has been pumped up as vapor by the sun's heat; while the combustion of coal from which steam-power is derived reproduces, as active force, or "energy," the sunshine that exerted itself during the Carboniferous period in dissociating carbonic acid and water into the hydrocarbons of coal and the oxygen of the atmosphere, whose recombination gives forth heat and light. And, if we look still further back for the source of the sun's radiant energy, we should find it, perhaps, in the progressive consolidation of the primeval "fire-mist"—nebular matter.

But whence nebular matter? And whence the force which draws its particles together, and which manifests itself as light and heat during their consolidation? Here we come to a wall, to the other side of which we seem at present to have no access.

But is there no other side? Does not the whole course of the preceding inquiry show the unsatisfaction (if I may revive an obsolete word) of resting in any inherent "potency" of matter as the ultima ratio of the existing cosmos? If we think the man foolish who supposes the main shaft of a cotton-mill to turn of itself merely because he sees it apparently end in a wall which conceals from him the source of its motive power, are we not really chargeable with the like folly if we attribute self-motion to the ultimate molecules of matter, merely because the power that moves them is hid from our sight? The mere physicist may see no possible way further. But there is a philosophy which has fully as true and as broad a basis in man's psychical experience as can be claimed for the fabric of physical science; and, in the admirable words of the great master I have already quoted (Sir John Herschel, in his "Familiar Lectures on Scientific Subjects," p. 460), I shall sum up an argument which this paper is intended rather to illustrate and enforce by an appeal to the familiar facts of consciousness than to present in strict logical form:

In the mental sense of effort, clear to the apprehension of every one who has ever performed a voluntary act, which is present at the instant when the determination to do a thing is carried out into the act of doing it, we have a consciousness of immediate and personal causation which can not be disputed