202 ' OCCCLTATION OF A STAR BY MARS. 



Occultat:on of a Had this obfervation been made under favourable circum- 

 sl«etMats. dances, the diameter of Mars mighl probabl)' have been de- 

 duced from it with a great degree of accuracy ; although fuch 

 determination would always have been fubje6l to uncertainty, 

 • from the eftefis of the refraction of his atmofphere, which it 



is impoffible to eftimate, but which feems, from fome circum- 

 ilances, to be very confiderable. 



In the prefent obfervation, two other caufes of uncertainty 

 txift. The firft is, the doubt of the duration of the occulta- 

 tion ; the fecond, the poffibility of miftake in the eftimaticn 

 of the difiance of the ftar from the centre of Mars. 



The firft of thefe caufes cannot induce an error of more than 

 two thirds of a fecond in the eftimate of his diameter. Pro- 

 bably the error is not more than half that quantity. 



The probable error arifing from the fecond caufe, may 

 amount to about the fame quantity as the former. Of this 

 I formed as near a judgment as I could, by drawing Figure 2, 

 Plate XJI. and placing the path of the ftar at its utmoft limits 

 of diftance from the centre by my eye. If the effecis of both 

 thefe caufes lie the fame way, an error of a fecond and an 

 half in the determination of the diameter of Mars, may pof- 

 fiblyarife; but it is highly probable, that the error is very 

 much lefs than that quantity, putting the etfeds of refraction 

 out of the queftion. 



Having, therefore, to the bell of my power, compared all 

 the circumftances of the obfervation, and the efiecls oF the 

 above caufes; I confider the equatorial diameler of Mars, as 

 refulling from it, to be at that time 14'^5-: and as the diftance 

 of Mars from the Eartii was llun 0.74456, the Sun*s mean 

 diftance being 1, the diameter of Mars at the mean diftance 

 j of the Sun, would be 10''.8. Dr. Herfchel, in iiis laft paper 



on Mars, makes it 9''.H. 



As all the caufes of error in my obfervation, tend rather to 

 dirainifli than increafe the fize of the planet, I am furprifed 

 that I ftiould have exceeded his eftimate ; which is, however, 

 lefs than that of former aftronomers. 



This, Sir, is all that has occurred to me on the fubjed of 

 this very uncommon obfervation. 



You will perceive, that I have not, in my eftimations of the 

 diameter of Mars, or of the diftances of the ftar from his 

 limb, made any mention of the fpheroidical figure of the pla« 

 net. Thediifercnces in the refults would be perfeClly infen- 



fible. 



