4. There are rivers that flow for hundreds of miles towards the level of the sea without falling more than a few feet - notably, the Nile, which, in a thousand miles, falls but a foot. A level expanse of this extent is quite incompatible with the idea of the Earth's "convexity." It is, therefore, a reasonable proof that Earth is not a globe.
The premise is false. There is no river that flows for hundreds of miles to the sea without falling more than a few feet.
The proof fails even further in that it does not support its claim of “quite incompatible”.