And the arithmetical complement of this lad loga- 
rithm, which is — 2.2212046, is the log. tangent of 
the Moon’s mean horizontal parallax at the equator; 
which therefore, is 57' 12", 34. 
Such would be the diftance of the Earth’s and 
Moon’s centers, were the Earth immoveable : but it 
is fomewhat increafed by their revolution round their 
common centre of gravity. 
Writing x 1 for that diftance, divided by the 
centre of gravity in the ratio of x to 1 ; imagine a 
fphere of the fame dimenlions as our earth, placed 
at that centre, to exert the fame attradive force on 
the Moon as our Earth adually does, the periodic 
time remaining unaltered : then mud the denfity of 
this fphere be diminished in the ratio of x % to x -\- 1 r, 
that its nearer diftance from the Moon may be com- 
pensated by the defed of denfity and attradive force. 
If now an inhabitant of the fiditious earth were fup- 
pofed to compute its diftance from the Moon, in 
the manner juft now (hewn 3 the quantities V and 
D would be the fame as in the former calculation ; 
but his j would be to our F, as x" to x 4- 1\ and 
thence, his x would be to our X as x r to x-|-i f , 
This is the diftance from the fiditious Earth, or 
from the common centre of gravity; but (T) the 
III. 
2 
that is, x — - 4. x X. 
v J r J 
.*• -(- r 
diftance from our Earth, is 
I 
X — — |f-xX, greater, 
A- 4 - I 1 3 0 
as 
