[ 5i3 ] 
As you have not fet down your firft obfervations 
on the fuperior or northern limb of Mars, as Mon- 
fieur De la Caille mention’d, that he would do on his 
part ; I have been obliged to fuppofe the apparent 
diameter of Mars known ; and, in the ufe of your 
hr ft obfervations, I have confidered it, as, or a little lefs 
than, you found it, when you began toobferve. 
The firft obfervation, which you made correfpond- 
ing with that of Monfieur De la Caille, is of Auguft 
31. in the morning, on which day you found, that 
the center of Mars in the meridian was 1 T 2 1" fouth 
of dar 33 of Pifces. If we deduct 13" for the 
apparent femidiameter of Mars at that time, ii' 8" 
will be the difference of the declination of the north- 
ern limb of Mars and the dar in the meridian of 
Greenwich. The diurnal variation of Mars in the 
declination was, at that time, 4' 47" ; whence we may 
conclude it to be 14" 48'" for i h 14', to be fubtradted 
from the didance obferved at Greenwich, in order 
to reduce it to what it would have been in the meri- 
dian of the Cape : So that, by your obfervation, the 
northern limb of Mars would have been fouth of the 
dar 10' 53" 12". MonfieurDe laCaille found, that 
dar north of the northern limb of Mars io' 18" 24'". 
The difference therefore is 34" 48'", for the lum of the 
parallaxes of the height of Mars, on that day, in the 
meridian of Greenwich and the Cape. 
Here follows what I have found, with refped to 
the other days. 
Sept. 4. in the morning, at Greenwich, Ri- t /; n/ 
gel fouth of the centre of Mars . . 7 yo 30 
Semidiameter of Mars . . . , o 1 3 00 
T tt 
Riget 
