ON THE PLANET VESTA. 259 



from E to R, and hollowed down to i- of an inch, so as to fit Description of 



..... a mercurial 



the bottom of the cylinder. pendulum. 



L is the bottom of the rod, and one. inch of the end of it is 

 made into a screw, that has forty threads in an inch. The 

 ri it K is J of an inch deep, and the diameter of its circle 

 from m to n is 1 T %- inch, having the upper edge divided into 28 

 equal parts, and figured 0, 1, 2, 3, or at each 7th division. 

 £ach of these divisions is very nearly equal to l" in 24 hours. 



The quantity of quicksilver required is between 10 and 11 

 lbs. It should fill the glass cylinder up to P, being 6'"4 inches 

 from the bottom of the glass, measured internally. Fig. 2 is 

 the cover of the glass cylinder, and fig. 3 the bottom of the 

 frame, that supports the cylinder, both viewed vertically. 



If with this pendulum the clock be found to go right with 

 the thermometer at 30°, and loses l" in 24 .hours with the 

 thermometer at 90°> it will be remedied by adding 10 oz. of 

 quicksilver; and if the reverse by taking out that quantity. 



The rod should be -J- of an inch thick, and £ of an inch 

 wide. The spring should be an inch long, and pretty stiff. 



IV. 



'Observations on the Nature of the new celestial Body discovered 

 by Dr. Olbers, and of the Comet which was expected to 

 appear last January in its return from the Sun. By Wil- 

 liam Herschel, L. L. D. F. R. S.* 



JL HE late discovery of an additional body belonging to the Account of 

 solar system by Dr. Olbers having been communicated to me l J) e "^^"ril 

 the 20th of April, an event of such consequence engaged my 20th, 1807. 

 immediate attention. In the evening of the same day I tried 

 to discover its situation by the information I had obtained of 

 its motion ; but the brightness of the moon, which was near 

 the full, and at no great distance from the object for which I 

 looked, would not permit a star of even the 5th magnitude to 

 be seen; and it was not till the 24th, that a tolerable view 



* Philos. Trans, for 1807, P. II, p» 260. 



S 2 could 



