Which of these images shows a vertical windmill? The other is a horizontal windmill. There is no right answer.
The first image shows the earliest variety of windmill, which developed in Persia around the eigth century CE. They were built in natural windtunnels, the wind turning sails mounted on a vertical axle. The axle turned the millstones directly; no gearing was involved. (I can't find any really clear images of this kind of mill, but these might help make a bit more sense of what's happening in the drawing above.)
The second image shows a tower mill, a kind of mill which developed in the fourteenth century, and which derived from its antecedent, the post mill. The post mill developed in the eleventh century, in England. The post mill's entire structure pivots around a central support pole, while with the tower mill, only the building's cap pivots. Post mills were thus constructed out of wood, a lighter material, while tower mills generally came to be built out of stone, because the building's weight did not need to be supported on a single post. With both post mills and tower mills, the sails turn a horizontal post, with gearing to transfer the circular motion of the sails and post to the millstones. The image is a detail of the illustration of the city of Rhodes in the Nuremberg Chronicle.
Every scholar whose work I have read on the early history of windmills is consistent in which type of mill they refer to as vertical and which is horizontal, but they are not at all consistent as to which kind of mill is a vertical one and which is a horizontal one. The majority of historians call the post mill and tower mill vertical mills; but not all historians. Part of the problem lies in modern-day terminology, where the mills structured most like the post mill (i.e., with a horizontal axis) are regularly called horizontal mills. Most historians look at the overall building structure, while most modern mill builders look at the axle around which the sails turn when calling mills one thing or the other.
In my dissertation, I've dodged the entire question by referring to mills as having either a horizontal or vertical axle; that, at least, can be universally agreed upon.
I was wondering what other people, coming to the problem and the pictures for the first time, might choose as best embodying verticality or horizontality.