...you have to make trial and assay what force it is that moves the weight along a level road — that is, to assay the nature of its friction; and, not knowing it, nor wishing to test it, you propose a difficulty in your own way and go varying it according to the obliquity of the road along which the weight is to be dragged. For different obliquities make different labours at its contact, and the reason is that if the weight to be moved is set on flat ground and is to be dragged over it, certainly that weight will be in the first power of difficulty, because it all rests upon the ground and nothing upon the cord that is to move it. But if you wish to pull it up a very steep road, all the weight that it gives of itself to the cord that sustains it is subtracted from the contact of its friction. But another, more evident reason must be shown. You know that in pulling it straight up, grazing and touching a wall somewhat, that weight is almost entirely on the cord that pulls it, and little remains on the wall, where it rubs.