I keep rereading the sections “why the ship’s hull disappears before the masthead” and “perspectives at sea” and though it sounds like he’s describing perspective as I understand it, he’s applying it in a way that is nonsensical to me. I get his argument about equidistant lines, but I can’t for the life of me deduce how he’s demarcing the horizon.
Each example, the surface appears to slope up to eye level, but then run parallel to (or coincident upon)the eyeline.
But how is that point figured? What’s happening here?
What determines the point at which the ground stops its apparent upward slope? Where does that H point that marks the horizon occur? Is dependent and how far above you the object lost to the horizon is? In other words, there is no horizon point. It’s a variable. The tops of tree are lost to the “horizon” further away than the trunks are?
What is determining where the red line appears to stop sloping upward and the blue line is level?
Is H variable, even if I’m not changing my height over the ground?
The key for understanding this idea of perspective is the introduction to the section "PERSPECTIVE ON THE SEA". There he describes some observations he made. Basically he observed that far away people seem to melt with the street. This effect is obviously the consequence of the limited optical resolution of our eyes. But for him this is the key to understand perspective. The limiting angle of optical resolution is something like 1°, but for him this is the angle of perspective lines relative to ground going away from the vanishing point.
Regarding any explanation about the vanishing point, this point is always at eye level. The consequence of this is, that point most far away to be observed is always at eye-level (anything beyond the vanishing point is to small to be visible). Within in this framework this is logical consequence, there is no other possibility. And cause the angle is fixed, the the distance to the vanishing point is not fixed. It moves away with your elevation, it comes closer if you go down.
Therefor, if you are at the sea, the point most far away is the horizon. Therefor the horizon is always at eye-level. There is no other option within this model.
Of course, all this is based on misunderstandings of optics, oversimplified drawings and so on. The biggest drawback is, that he explains everything only in a vertical plane. But perspective works in all directions, it is isotropic. Would you apply his model also to the horizontal plane, everything would look like if we would live in a tunnel. Obviously he never thought about all the consequences of his model. It is also not quite clear why the sun is visible anyway in England, because it is always further away than the horizon. OK, it is bigger and higher in the sky, but I guess that if you would put numbers into his model, you would find lots of contradictions regarding height, size and distance.
But anyway, this what comes closest to a theory in the whole book and is the most central part of all his "experiments", observations and explanations. And therefor this obsession of the flat-earth believers with this "horizon at eye-level" claim. If this fails, most of the other stuff will also fail. It's the house of cards everything is build upon. So they will never accept anything, that is in contradiction to this claim.