172 HISTORY OF PHYSICAL ASTRONOMY. 



ists we have mentioned, has invented a method of dividing the circle 

 still superior to the former ones ; indeeu, one which is theoretically 

 perfect, and practically capable of consummate accuracy. In this way, 

 circles have been constructed for Greenwich, Armagh, Cambridge, and 

 many other places; and probably this method, carefully applied, offers 

 to the astronomer as much exactness as his other implements allow 

 him to receive ; but the slightest casualty happening to such an instru- 

 ment, after it has been constructed, or any doubt whether the method 

 of graduation has been rightly applied, makes it unfit for the jealous 

 scrupulosity of modern astronomy. 



The English artists sought to attain accurate measurements by con- 

 tinued bisection and other aliquot subdivision of the limb of their 

 circle ; but Mayer proposed to obtain this end otherwise, by repeating 

 the measure on different parts of the circumference till the error of the 

 division becomes unimportant, instead of attempting to divide an in- 

 strument without error. This invention of the Repeating Circle was 

 zealously adopted by the French, and the relative superiority of the 

 rival methods is still a matter of difference of opinion. 



[2d Ed.] [In the series of these great astronomical mechanists, we 

 must also reckon George Reich enbach. He was born Aug. 24, 1772, 

 at Durlach ; became Lieutenant of Artillery in the Bavarian service 

 in 1794; (Salinenrath) Commissioner of Salt-works in 1811 ; and in 

 1820, First Commissioner of Water-works and Roads. He became, 

 with Fraunhofer, the ornament of the mechanical and optical Institute 

 erected in 1805 at Benedictbeuern by Utzschneider; and his astro- 

 nomical instruments, meridian circles, transit instruments, equatorials, 

 heliometers, make an epoch in Observing Astronomy. His contriv- 

 ances in the Salt-works at Berchtesgaden and Reichenhall, in the Arms 

 Manufactory at Amberg, and in the works for boring cannon at Vienna, 

 are enduring monuments of his rare mechanical talent. He died May 

 21, 1826, at Munich.] 



2. Clocks. The improvements in the measures of space require cor- 

 responding improvements in the measure of time. The beginning of 

 any thing which we can call accuracy, in this subject, was the applica- 

 tion of the Pendulum to clocks, by Huyghens, in 1656. That the 

 successive oscillations of a pendulum occupy equal times, had been 

 noticed by Galileo ; but : n order to take advantage of this property, 

 the pendulum must be connected with machinery by which its motion 

 is kept from languishing, and by which the number of its swings is 

 recorded. By inventing such machinery, Huyghens at once obtained 



