most modern screw presses allow for a continuous flow of material by surrounding the screw with a screen, which also helps to avoid the build up of a layer of solid material on the screen. the first stone must be dropped into the hole which was just emptied. although the fourth is not as much a main character as the first three, it is still a main character. their strength is such that the ability to crush ice is an expected feature. (the number  could be replaced by any number on the upper half-plane, but  is the most convenient choice.) 