Take, for instance, June 9th, 1868.

We find from 'Dietrichsen' that on this day (at noon) Mercury's R.A.is 6h.

53m.
23s.: and the sun's 5h.11m.31s.We need not trouble ourselves about the odd hours after noon, and thus we have Mercury's R.A.greater than the sun's by 1h.41m.52s.Now we will suppose that the observer has so fixed his uprights and the two rods, that the sun, seen from the fixed point of view, appears to pass the point of crossing of the rods at half-past seven, then Mercury will pass the cross-rod at 11m.52s.

past nine.

But where?
To learn this we must take out Mercury's declination, which is 24 deg.