Index.of: Jpg Mast Receding A Distant Ship Smoke On The Horizon?

Receding a distant ship smoke on the horizon? - index.of: jpg mast

Because the Earth is round, there is no observable horizon.
... http://upload.wikimedia.org/wikipedia/en

As the pressure of the atmosphere should appear on the surface of the earth is flat and disappear from the horizon?

Refractive index of standard air at sea level
n = 1.00029