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HISTORICAL VIEW OF THE

sufficient for the purposes of astronomy as it then stood.The system was, therefore, adopted by astronomers as aconvenient one for practice, even if they had beeninclined to question its physical truth. Nevertheless,the conception of the planetary world carrying on itsmotions by the aid of such mechanism, must havebeen a considerable effort even to the most powerfulimagination. Some illustration of the supererogatorydifficulty thus occasioned may he afforded, if we ob-serve, in passing through a wood, in how extremelycomplicated a manner the relative positions of the treesappear at each step to be continually changing; and byconsidering the difficulty with which the laws of theirapparent motions could be traced, if we were to attempt torefer these changes to a real motion of the trees, insteadof to that of the traveller. In order, however, to com-plete the illustration, we must yet further imagine whatwould be the increased complexity, if each of the treeshad a real motion of its own, besides their apparentmotions resulting from that of the observer; which isthe case in regard to the planets and the earth.

Such, however, was the Ptolemaic system, whichobtained an ascendancy for a longer series of ages, andover a larger portion of the world, than any other. Amonstrous complication of purely gratuitous hypotheses,which, to account for every new inequality, had to beentangled in new perplexities, and, to solve every freshdifficulty, had to he involved in more hopeless confusion ;and its votaries went on, in the language of the poet,—

“ To save appearances; and gird the sphereWith centric and excentric scribbled o’er,

Cycle and epicycle, orb in orb."

All this complexity being incurred solely to avoidimpugning the grand dogma, that “ uniform motionsbelong by their nature to celestial bodiesand the sub-lime principle, that “ circular motion is perfect andincorruptible.” Yet such was the system, not onlyadopted in that age by the contemporaries and fol-lowers of Ptolemy , but which actually held its ground