IMPROVEMENT IN TIME-PIECES. 203 



fible, if that element had been confidered ; and as I never Occultatlon of a 

 ha\'e been able to perceive the difference of his diameters, I n]zntt Mars, 

 -cajinot but fufpecl it to be lefs than Dr. Herfchel fuppofes. 

 I am, 



Dear Sir, 



Your obliged and faithful, 

 Maj/23, 1797. H. C. ENGLEFIELD. 



P. S. In the Connaiffunce des Terns for the jear VI. page 

 457, and year VII. page 428, are accounts of an obfervation 

 of this occultatlon, made at Aubenas, or Viviers (for it is not 

 certain at which of thefe places) by Mr. Flaugergue, member 

 of the National Inftitute. It is not clear from thefe accounts 

 that he adually faw the emerfion : at 15*58'25^^ apparent 

 time, he faw the ftar^ difiant from the limb of the planet only 

 by an interval equal to the fuppofed breadth of the dark part 

 of the difc of Mars, ahd it was extremely faint, but perfectly 

 white. Its poiilion was about 20® to the weft of the vertical, 

 and above the horizontal diameter. Mr. Flaugergue fuppofes 

 that the ftar paffed within a fecond of the northern limb of 

 Mars, and muft have been hidden ten minutes and an half. 

 , As the place of obfervation was in a lower latitude than 

 Xondon, the obferver there muft have feen the ftar pafs nearer 

 the centre of Mars than I did, the effeCl of parallax being lefs, 

 and the occultation muft in confequence have lafted longer : 

 but my obfervation proves, beyond a doubt, that the ftar 

 paffed much nearer the centre than Mr, Flaugergue fuppofes, 

 as I loft light of the ftar at leaft nineteen minutes and an half. 



IX. 



Letter from Mr. Ezekiel Walker, containing a confiderahle 

 Improvement in Time-Pieces by Mr. Barraud, with Rt» 

 marks. 



To Mr. NICHOLSON, 

 SIR, 



1 HE method of determining the longitude by time-keepers. The determlna- 

 poffeffes fome advantages fuperior to any other that has yet *^°" °^ '°"6*'^"''* 

 been propofed. In the firft place, this method is attended is^morTVafytS 



with ^^^ 1)6 oftener 



