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HISTORY OF PHYSICAL ASTRONOMY.

ments were similar to those of Tycho Brahe ; hut they were divided withgreater precision, and were more easily managed. The application of thevernier to the measurement of fractional magnitudes, was an improvementof which he did not fail to avail himself. This elegant method of subdi-vision was invented in the year 1031 by Pierre Vernier, a native ofFranche Compte*. Hevelius himself devised an ingenious method ofeffecting the same object by means of a tangent screw, working in theextremity of the movable radius upon which the pinnules were placed.Knowing the number of revolutions of the screw which were necessaryto transport the radius over the interval included between any two con-secutive points of division of the limb, it was hence easy, by countingthe revolutions and parts of [a revolution of the screw, in moving theradius from any point on the limb to the nearest point of division, toascertain the magnitude of the space included between the two points.This must be considered as one of the earliest of those mechanical con-trivances that have been devised for measuring the fractional parts ofdivided instruments. Hevelius , by refusing to employ the telescope inhis observations, was unable to take due advantage of this ingeniousmethod of subdivision; but it was applied with complete success inseveral instances in the following century.

In 1670 the observatory of Hevelius was totally consumed by fire.All his instruments and manuscripts, and, with the exception of a fewcopies, the whole edition of the second part of his great work, theMachina Ccclcstis, comprehending the numerous observations which hehad made during his long career, unfortunately perished in the flames.The aged astronomer bore this calamity with great fortitude; and, insteadof abandoning himself to despair, he recommenced his labours withrenewed energy j-. One of the most remarkable of his works is acatalogue of the places of upwards of 1500 stars, which was published afew years after his death. The great ability of Hevelius , as an observer,would have rendered this catalogue a valuable record of the state of theheavens in his time, if, in making his observations, he had availed him-self of telescopic sights, which came into general use during the latterpart of his career. Unfortunately, however, he persevered to the end ofhis life in rejecting such appliances, adhering to the use of simple pin-nules, under the impression of their affording a more trustworthy indica-tion of the true direction in which a celestial body appears.

With the labours of Hevelius closes the ancient school of astronomicalobservation. By constructing instruments of very large size, and em-ploying ingenious methods of subdivision, astronomers had succeeded indetermining the apparent position of a celestial body within about V ofthe truth. If the magnitude of the instrument in any instance, however,exceeded a certain limit, it obviously became unmanageable ; and besides,its greater tendency to derangement from its own weight more than coun-terbalanced the advantage gained by enlarging the scale of division. But

* A method somewhat similar in principle, hut more complicated in practice, hadbeen invented by Nonius, a Portuguese mathematician, about the middle of the six-teenth century. Tycho Brahe used it in the eariy part of his career, but he soon aban-doned it as too troublesome, adopting the method of transversals in its stead.

f What Hevelius regretted on this occasion more than all his most precious effectswere his researches on the planetary motions, and his notes on the stars, which he hadbeen making throughout the long period of his astronomical career. Several years after-wards he assures the reader, in one of his works, that the recollection of their loss neverfailed to bring tears into his eyes.