MATHEMATICAL PAPERS. tgs 
may make ufe of the tranfits acrofs the horizontal wire, to de- 
duce the difference of altitude, becaufe the motion of the two 
planets, vertically, was become lefs flow, when this obferva-- 
tion was made. 
If we would, in the firft obfervatiop, verify the inftant of 
the tranfit of the apparent lower horn acrofs the vertical wire, 
or know, in the fecond obfervation, the inftant of the tranfit 
of the limb of the moen acrofs the fame wire, we may eafily 
calculate them, with the affiftance of the quantities already 
known. It will, perhaps, be ufeful to make this calculation, 
that we may be in a condition of determining the variation, 
which muft have taken place, in the pofition of the two pla- 
nets, obferved relatively the one to the other, during the fpace 
ef time which intervened, between their tranfits semis the. 
fame wire.. 
