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 would do as well, either of chemical explosion or vegetable growth, or of expansion or contraction in innumerable forms; but gravitation happens to be simple and convenient. As the human meteoroid approached the sun or centre of attractive force, it may be supposed to have doubled its speed and energy every five-and-twenty years.

Behind the year 1400, the process certainly went on, but the progress became so slight as to be hardly measureable. What was gained in the east or elsewhere, cannot be known; but forces, called loosely Greek fire and gunpowder, came into use in the west, as well as instruments like the compass and blow-pipe; while metaphysics and theology acted as violent stimulants to mind. An architect might detect a sequence between the Church of St. Peter's at Rome, the Amiens Cathedral, the Duomo at Pisa, San Marco at Venice, Sancta Sofia at Constantinople and the churches at Ravenna. All the historian dares affirm is that a sequence is manifestly there, and he has a right to carry back his ratio, to represent the fact, without assuming its numerical correctness. On the human mind as a moving body, the break in acceleration in the middle-ages is only apparent; the attraction worked through shifting forms of force, as the sun works by light or heat, electricity, gravitation, or what not, on different organs with different sensibilities, but with invariable law.

The science of prehistoric man has no value except to prove that the law went back into indefinite antiquity. A stone arrowhead is as convincing as a steam-engine. The values were as clear a hundred thousand years ago as now, and extended equally over the whole world. The motion at last became infinitely small, but cannot be proved to have stopped. To evolutionists may be left the processes of evolution; to historians the single interest is the law of reaction between force and force,—between mind and nature,—the law of progress.

The great division of history into phases by Turgot and Comte first affirmed this law in its outlines by asserting the unity of progress, for a mere phase interrupts no growth, and nature shows innumerable such phases. The development of coal-power in the nineteenth century furnished the first means of assigning closer values to the elements; and the appearance of supersensual forces towards 1900 made this calculation a pressing necessity; since the next step became infinitely serious.

A law of acceleration, definite and constant as any law of mechanics,