It is difficult, indeed, to understand how so thorough an astronomer as the late Admiral Smyth could have called the passage in which these lines occur one of the finest bursts of poetry in our language, except on the principle cleverly cited by Waller when Charles II. upbraided him for the warmth of his panegyric on Cromwell, that 'poets succeed better with fiction than with truth.' Macaulay, though not an astronomer, speaks more justly of the passage in saying that this single passage contains more inaccuracies than can be found in all Wordsworth's 'Excursion.' [41] It may be necessary to throw in here a few words of explanation, lest the non-astronomical reader should run away with the idea that the so-called exact science is a very inexact science indeed, so far as comets are concerned. The comet of 1680 was one of those which travel on a very eccentric orbit. Coming, indeed, from out depths many times more remote than the path even of the remotest planet, Neptune, this comet approached nearer to the sun than any which astronomers have ever seen, except only the comet of 1843. When at its nearest its nucleus was only a sixth part of the sun's diameter from his surface. Thus the part of the comet's orbit along which astronomers traced its motion was only a small part at one end of an enormously long oval, and very slight errors of observation were sufficient to produce very large errors in the determination of the nature of the comet's orbit. Encke admitted that the period might, so far as the comparatively imperfect observations made in 1680 were concerned, be any whatever, from 805 years to many millions of years, or even to infinity--that is, the comet might have a path not re-entering into itself, but carrying the comet for ever away from the sun after its one visit to our system. [42] For a portion of the passages which I have quoted in this essay I am indebted to Guillemin's 'Treatise on Comets,' a useful contribution to the literature of the subject, though somewhat inadequate so far as exposition is concerned.