It was necessary in that age, if he were to convince others, to show more than the principle of his engine,--he must show also something of its effects; turn a mill without wind or water, or set in motion some mimic vehicle without other force than that the contrivance itself supplied.
And here, at every step, new obstacles arose.

It was the misfortune to science in those days, not only that all books and mathematical instruments were enormously dear, but that the students, still struggling into light, through the glorious delusions of alchemy and mysticism, imagined that, even in simple practical operations, there were peculiar virtues in virgin gold and certain precious stones.

A link in the process upon which Adam was engaged failed him; his ingenuity was baffled, his work stood still; and in poring again and again over the learned manuscripts--alas! now lost--in which certain German doctors had sought to explain the pregnant hints of Roger Bacon, he found it inculcated that the axle of a certain wheel must be composed of a diamond.

Now, in truth, it so happened that Adam's contrivance, which (even without the appliances which were added in illustration of the theory) was infinitely more complicated than modern research has found necessary, did not even require the wheel in question, much less the absent diamond; it happened, also, that his understanding, which, though so obtuse in common life, was in these matters astonishingly clear, could not trace any mathematical operations by which the diamond axle would in the least correct the difficulty that had suddenly started up; and yet the accursed diamond began to haunt him,--the German authority was so positive on the point, and that authority had in many respects been accurate.

Nor was this all,--the diamond was to be no vulgar diamond; it was to be endowed, by talismanic skill, with certain properties and virtues; it was to be for a certain number of hours exposed to the rays of the full moon; it was to be washed in a primitive and wondrous elixir, the making of which consumed no little of the finest gold.