6CCULTATI0N OF A STAR BY MARS^ 19^ 



Four fince ibe invention of telefcopes. Of thefe, one only is 

 by Mars, and that fo long ago as about the year 1630, by 

 Gaflendi. The curious and very important conjun6lion of 

 Mars with pli aquarii, in 06lober 1672, whence Caffini had 

 hoped to deduce, with great accuracy, the parallax of Mars 

 and the Sun, was miffed by bad weather at the moment of 

 the occultation. Since that time, I do not know that any ob- 

 fervation of this kind has been made. 



The night of April 17, 1796, being very clear, I was led Account of the 

 by mere accident to diredl my telefcope to Mars, about two*'^^^'^^^^°"* 

 o'clock in the morning ; and I faw with equal pleafure and 

 furprife, a liar, about three of his diameters to the eaft of 

 him, and in a line perpendicular to his horns, he being then 

 gibbous. It was evident that an occultation would take place, 

 and I prepared every thing for the obfervation. 



The telefcope I ufed is an achromatic of Ramfden, the InftrumentSs 

 aperture 2.7Ji, with a (ingle eye-glafs, magnifying 100 times. 

 The clock is a very good regulator, with a wooden pendulum, 

 and to be depended on to lefs than 2 feconds in the 24 hours. « 



The obfervations for the time were made with an excellent fix 

 incii Hadley's fextant of Ramfden's, and an artificial horizon 

 of mercury. 



The pofition of Peterfham, as deduced by a furvey made Place of the oWi 

 by me fome years fince, from the Royal Obfervatory at Kew, 

 which was one of the points fettled by General Roy in his 

 great furvey, U U \Q!' in time weft of the Obfervatory at 

 Greenwich in longitude j and its latitude by the fame furvey 

 51°26'31.^ 



To return to the obfervation. The Iky, during the whole Collateral cir- 

 time, was perfedly free from clouds, but was remarkably full «"n^^a"««« 

 of vapours, which at times produced a greater undulation in" 

 the limb of Mars, than I ahnoft ever faw. The wind was in 

 the eaft, and probably the columns of warm air blown from 

 London, mixed very irregularly with the atmofphere of Pe- 

 terfham. This ftate of the air, and the low altitude of Mars, 

 whofe fouthern declination was 23**i, rendered it impoflible to 

 diflinguilh with certainty the ftar, from the protuberances in 

 the waving limb of Mars at the moment of immerfion, though 

 the brilliant and filvery light of the ftar contrafted ftrongly 

 with the dull red hue of the planet. The obfervation of the 

 immerfion of the ftar is therefore uncertain, to near a minute 



