«*ct. iv.l DISSERTATION SECOND. 153 



discovered was complete. The reasoning by which he 

 convinced himself, is a proof how slowly men are cured of 

 their prejudices, even with (he best talents and the best in- 

 formation. The planets, primary and secondary, thus made 

 up twelve, the double of six, the first of the perfect num- 

 bers. In 1671, however, Cassini discovered another satel- 

 lite, and afterwards three more, making five in all, which the 

 more perfect telescopes of Dr. Herschell have lately aug- 

 mented to seven. 



To the genius of Huygens astronomy is indebted for an 

 addition to its apparatus, hardly less essential than the 

 quadrant and the telescope. An accurate measure of time 

 is of use even in the ordinary business of life, but to the 

 astronomer is infinitely valuable. The dates of his obser- 

 vations, and an accurate estimate of the time elapsed be- 

 tween them, is necessary, in order to make them lead to any 

 useful consequences. Besides this, the only way of mea- 

 suring with accuracy those arches in the heavens, which 

 extend from east to west, or which are parallel to the equa- 

 tor, depends on the earth's rotation, because such an arch 

 bears the same proportion to the entire circumference of a 

 circle, that the time of its passage under the meridian bears 

 to an entire day. The reckoning of time thus furnishes 

 the best measure of position, as determined by arches 

 parallel to the equator, whether on the earth or in the hea- 

 vens. 



Though the pendulum afforded a measure of time, in 

 itself of the greatest exactness, the means of continuing its 

 motion, without disturbing the time of its vibrations, was 

 yet required to be found, and this, by means of the clock, 

 Huygens contrived most ingeniously to effect. Each vi- 

 bration of the pendulum, by means of an arm at right an- 

 gles to it, allows the tooth of a wheel to escape, the wheel 

 being put in motion by a weight. The wheel is socontriv- 



