65)  “Also Quoting Dr. Rowbotham, “On the shore near Waterloo, a few miles to the north of Liverpool, a good telescope was fixed, at an elevation of 6 feet above the water. It was directed to a large steamer, just leaving the River Mersey, and sailing out to Dublin. Gradually the mast-head of the receding vessel came nearer to the horizon, until, at length, after more than four hours had elapsed, it disappeared. The ordinary rate of sailing of the Dublin steamers was fully eight miles an hour; so that the vessel would be, at least, thirty-two miles distant when the mast-head came to the horizon. The 6 feet of elevation of the telescope would require three miles to be deducted for convexity, which would leave twenty-nine miles, the square of which, multiplied by 8 inches, gives 560 feet; deducting 80 feet for the height of the main-mast, and we find that, according to the doctrine of rotundity, the mast-head of the outward bound steamer should have been 480 feet below the horizon. Many other experiments of this kind have been made upon sea-going steamers, and always with results entirely incompatible with the theory that the earth is a globe.”

Another experiment, similar to the first Bedford level experiment. Basically, he watched a steamer, which is probably a  large steamboat, `sail' towards the horizon. Because it is hilarious, we will use a flat-earth video. It should be at time=8:02.

https://youtu.be/cYfB0B16VQA

Look closely;  he will start comparing the two times. He apparently claims that this is atmospheric refraction, not the horizon. However, look at the bottom of the ship! It is clearly disappearing below the horizon. So, an image that shows a ship disappearing behind the horizon is used as evidence that it doesn't.”

For a better quality, uncut version of the same video of a ship moving below the horizon, look here.

1) Every sailor knows that the speed of a vessel and its true speed over ground are entirely different things. Due to currents they can have completely different values. Mr Rowbotham did not factor this into his calculations.

2) Atmospheric refraction is a big factor and can make objects visible that geometrically are far beyond the horizon, especially when you look out on the sea on a sunny day. Again, this was not factored in.

3) Assuming a mast-height of 80 feet (roundabout 25m) and an observing position at 6 feet of elevation (roundabout 2m), you should be able to see the mast-head up until at least 23km away geometrically.

http://www.ringbell.co.uk/info/hdist.htm

Therefore, factoring in refraction and true speed of the vessel, a steamboat sailing away for four hours can be well within your visible range.