The photo above shows several kayakers in a harbor. Each kayaker is overlaid with a graphic of a kayaker's silhouette multiplied three times to reach the horizon. Each of their boats is sitting about eight feet below the horizon. That's because the viewer is standing with the camera about eight feet above the water.
If you were drawing this scene, and wanted to add another kayak, whether close or far, you would have to make sure its waterline was also a little over three sitting man-heights below the horizon.
A boat placed at a given distance below the horizon must be of a certain size, and a boat of a certain size must be a certain distance below the horizon.
If you get this wrong, the harbor will be filled with toy boats. A lot of artists have made this mistake, even a lot of famous artists, such as Frank W. Benson (1862-1951). Mr. Benson has established that observer is about four feet above the water, because the horizon crosses about one head higher than the standing boy.
But if we apply that measure to the rowboat to the left, we have a problem. That rowboat now looks four feet long. If you stand it on end from bow to stern, it would just reach from the waterline to the horizon. The rower would have to be just one foot tall. And the figure in the the sailboat at right would have to be the size of a GI Joe.
Edit: Since so many people asked about the Loomis explanation of this principle using figures on land, I have added them here. Thanks, Ben!
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