TranJU of Mercury. 43 1 



ASTRONOMY. 



The tranfit of Mercury over the fun's difk is a phenome- 

 non of the utmoft importance to aftronomy, as it affords the 

 means of improving the theory and tahles of that planet ; hut 

 the late phenomenon of this kind, which took place on the 

 7th of May, was deferving of the more attention, as a tranlit 

 of Mercury at the defcending node of his orbit had never 

 been obferved in its whole duration. Thefe tranfits, in gene- 

 ral, are much more uncommon than thofe at the afcendincr 

 nodes. Ever fince mankind began to view the heavens 

 Mercury had been obferved only three times under the like 

 circumftances, and unfortunately each obfervation was in- 

 complete. In the year 1661 the ingrefs of the planet into 

 the fun was feen, but not its egrefs; and in the years 1753 

 and 17^6 the egrefs only was obferved. It is much, there- 

 fore, to be wifhed that the weather had been more favour- 

 able during the late tranfit, as the fame phenomenon, under 

 the like circumftances, will not occur before the year 1 3-. 

 According to an obfervation made by a gentleman at Green- 

 wich, on this tranfit, on Tuefday the 7th, with a feven feet 

 telefcope by Dr. Herfchel, the aperture of which was con- 

 tracted, the internal contact with the SW limb of the fun 

 happened at 4 h aS' 32", and the egrefs at 4 h 31' 2 \' p. m. 

 mean time. The ingrefs was not feen on account of the cloudi- 

 nefj of the morning. 



Mercury has at all times afforded much occupation to 

 aftronomersj becaufe, to obferve that planet, is a matter of 

 confiderable difficulty. The great Copernicus died without 

 ever having feen it, and therefore could believe only in its 

 exiftence. The celebrated Maehiinus, the tutor of the im- 

 mortal Kepler, ufed to fay that this planet was calculated only 

 to expofe aftronomers to the danger of lofing their reputation : 

 fo that when he knew of any one employed in tracing out its 

 intricate courfe, he advifed him to employ his time en fome 



better 



