;<j'^ M. DE Zach's JJlronomkal Obfervatlons, 



I only add an important remark upon the diameter of Mer- 

 ,'Cury, which the aftronomers fuppofed in this traafit=: 12'^. 



Let ABC reprefent the fun's difk ; in P an external in Q^an 

 internal conta<£t ; ANC the apparent path of Mercury over tlic 

 fun. 



The femi-diameter of the fun = 972^', this of Mercury in our 

 fuppofition = 6'% MN = 942'^ the leaft dlftances of the centers. 

 In the right-angled triangle MNP it is MP :=: 972'' + 6''rz 978'% 

 MQ=:972''-6''=966''; therefore NP will be foundr;26o'^ 

 and NCi= 2 1 o"' : now NP — NQr: PQ^=: $0^', which converted 

 into time gives V i\" for the time the diameter of Mercury em- 

 ployed to run over the limb of the fun ; but by the obfervations 

 of M, Wallot I find this time conftantly in both contads 

 5' 35^^; therefore V 14'' : ii." :: 5' '3,^" : 8^137, whicU 

 (hould be the diameter of Mercury ; and indeed M. Wallot^ 

 by an immediate meafure, taken with an excellent wire-micro- 

 meter, finds this apparent diameter not greater than 9"^ which 

 fufficiently (hews that this diameter fuppofed f wy the meg?^ 

 diftance is alfo too great. 



lam, &c.. 



