154 DISSERTATION SECOND. [pa*t i. 



ed, that the force with which it acts is just sufficient to 

 restore to the pendulum the motion which it had lost by the 

 resistance of the air, and the friction at the centre of mo- 

 tion. Thus the motion of the clock is continued without 

 an/ diminution of its uniformity, for any length of time. 



The telescope had not yet served astronomy in all the 

 capacities in which it could be useful. Huygens, of whose 

 inventive genius the history of science has so much to re- 

 cord, applied it to the measurement of small angles, form- 

 ing it into the instrument which has since been called a 

 micrometer. By introducing into the focus of the teles- 

 cope a round aperture of a given size, he contrived to 

 measure the angle which that aperture subtended to the 

 eye, by observing the time that a star placed near the 

 equator required to traverse it. When the angle subtend- 

 ed by any other object in the telescope was to be measur- 

 ed, he introduced into the focus a thin piece of metal, just 

 sufficient to cover the object in the focus. The proportion 

 of the breadth of this plate, to the diameter of the aperture 

 formerly measured, gave the angle subtended by the image 

 in the focus of the telescope. This contrivance is describ- 

 ed in the Syslejna Satumium, at the end. 



The telescope has farther contributed materially to the 

 accuracy of astronomical observation, by its application 

 to instruments used for measuring, not merely small an- 

 gles, but angles of any magnitude whatever. The te- 

 lescope here comes in place of the plain sights with 

 which the index or allidad of an instrument used to be 

 directed to an object, and this substitution has been ac- 

 companied with two advantages. The disk of a star 

 is never so well defined to the naked eye as it is in the 

 telescope. Besides, in using plain sights, the eye adapts 

 itself to the farther off of the two, in order that its 

 aperture may be distinctly seen. Whenever this ad- 



