<s id="A18-1.21.01">[21] The stretches of water, now, that are situated on not inclined planes, do not flow, but they are still, without inclining towards any side.</s>
<s id="A18-1.21.02">If they experience, however, even the slightest incline, then they all flow towards that side, so that not even the smallest part of the water remains on it, unless there might be depressions in the plane, so that small parts would remain in the hollow of these depressions, as sometimes occurs with vessels.</s>
<s id="A18-1.21.03">This happens, however, to the water because its parts are not joined, but rather are easily separable.</s>
<s id="A18-1.21.04">Since, however, the joined bodies are by nature not smooth on their surfaces and cannot easily be made even, the roughness of the bodies causes them to support one another and this again causes them to lean against one another like cogwheel mechanisms, so that they are prevented from it; for when they are numerous and tightly joined to each other through a mutual fusion, a united large force is necessary.</s>
<s id="A18-1.21.05">From experience one has now drawn the lesson; for one started to put under the "turtles" pieces of wood, whose surface is formed cylindrically, touching only a small part of the plane, which is why only the smallest friction occurs.Now one uses stakes, so the load can easily be moved on them, under the condition that the load is increased by the weight of the tool.</s>
<s id="A18-1.21.06">Others attach planed boards to the bottom, because of their smoothness, and smear them with fat, in order to smooth out the roughness on them, and then move the load with very little force.</s>
<s id="A18-1.21.07">As for the cylinders, they can, if they are heavy and lie on the ground so that only a single line touches the ground, be moved with ease, and as well the spheres that we already talked about.</s>
