152 M. deZach's AJlronomkal Objerv^Uions, 



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

 cury, which the aftronomers fuppofed in this tranfit=: 1 1^\ 



Let ABC reprefent the fun's di(k ; in P an external in Q^an 

 internal conta(fl; ANC the apparent path of Mercury over the 

 fun. 



Thefemi-diameterof the funir 972''^, this of Mercury in our 

 fuppofitionrr6^% MN = 942'^ the leaft diftances of the centers. 

 In the right-angled triangle MNP it is MP :=: 972'' -f 6'' = 978'', 

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

 and NQr: 2 1 o^'' : now NP— NQ=PQ^r: 50", which converted 

 into time gives 8' 14^^ 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 conflantly in both contacts 

 5' 35^^ > therefore 8' 14'' : 12'' :: 5^ 35'^ : 8^^137, which 

 {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 fhews that this diameter fuppofed 7'' ia the meaa 

 4iltance is alfo too great, 



I am, &c. 



