Hello,
We assume that the Earth is a perfect sphere (which it is not) and look at the cross section determined by these two  10 ft  structures:

Now we use radian measure, namely the equation:  θ = s/r

Since these structures are plumb, they lie perfectly on the radial lines of the Earth.  Thus,  θ is the same for the small and big circles, and we obtain:

Therefore the difference between these two arc lengths is,

So we wish to solve for s₁ knowing that s₂ - s₁ = 1 inch = 1/12 foot, and r₁ = r₂ + 10 feet.

What is the radius of the Earth in feet?  How far apart are the two structures?
Tyler