on ibe Planet Mars. 27% 
fhould be, will equally ’ afeat the wires of the micrometer, 
and give a true meafure notwithftanding ; and the compound 
eye-piece I ufed with the 20-feet reflector had likewife the fame 
advatitase, for it is conftruéted on the plan lately propofed by 
Mr. Ramsbden in the Philofophical Tranfactions*, which he 
was fo obliging a8 to cormunicate to. me about a twelve-month 
aso, and which I page eg adapted to my large micro- 
meters. | a 
On the fubject of the figure of Mars I ought to Jebbait 
alfo, that perhaps the meafures which were taken of its dia- 
meters during ‘the laft oppofition will enable us to afcer- 
tain its real fize with greater accuracy than has been done 
before. The micrometer which can diftinguifh with precifion 
_between the equatorial and polar diameters of this fmall pla- 
net, will certainly be adinitted as an evidence of confiderable 
confequence; and fince the refult of thefe ‘tmeafures i is pretty 
different from what former obfervations give fis; I fheuld not 
‘omit mentioning tt: 
We have feen that the equatorial diameter, on the day of 
the oppofition, meafured 22/7 35’”. The diftance of Mars 
from the carth at that time was . 404.57; the mean fees of 
the earth from the fun being i; therefore, 227"35" teduced 
to the fame diftance will be no more than 9” 8’”. 
I fhall conclude this fubje@ with a confideration relating to: 
the atmofphere of Mars. Dr. Smirx + reports an obfervation 
ef Cassini’s, where “a ftar in the water of Aquarius, at the 
‘* diftance of fix minutes from the difk of Mars, became fo 
“« faint before its occultation, that it could not be feen by the 
“‘ naked eye, nor with a 3-fect telefcope” It is not mene 
— * Vol. LXXIIL p. ote 
+ Optics, § 1096. 
tioned 
