let. iv. TychoBrahe, and Copernicus. bz

In this new system of Tycho’s, there is someingenuity, though but little conformity to truthand observation. For having rejected the diur-nal rotation of the earth on its axis, he wasobliged to retain the most sliocking part of thePtolemaic hypothesis, and to suppose that thewhole universe, to its farthest visible limits, wascarried by the primum mobile about the axis ofthe earth continually every day. But in this,however, he was abandoned by some of his fol-lowers, who chose rather to save this immenselabour to the spheres, by ascribing a diurnal mo-tion to the earth; on which account they weredistinguished by the name of Semi-Tychonics.

But though Tycho was not happy in esta-blishing a new system, he was yet of great useto Astronomy, by his diligence and exactness inmaking observations for a long series of years.Amongst other things, he discovered the re-fraction of the air, and determined the placesof a great number of the fixed stars, with anaccuracy unknown to the astronomers of formertimes. He likewise demonstrated, against theopinion which then prevailed, that the cometswere higher than the moon; and from his ob-servations on this, and the rest of the planets,the theories of their motions were afterwardscorrected and improved; so that for these ser-vices he will always be celebrated and esteemedby astronomers.

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