The Mishnah mentioned some sort of roller to spread plaster over rooftops. Why is this called מעגילה especially since it only called that way on the way down?
The same word מעגילה in this folio appears whether the roller is going up or going down.
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The gist of the passage is that when going up to the rooftop with the roller, you were working against gravity. Thus if the roller falls and hurts someone while you were working against gravity, you are not to be exiled from the land.
However, once you are on the rooftop, you now begin to leverage gravity to work for you. Because you are using gravity for your own benefit, you now incur the liability that gravity could pull something out of your control and hurt someone. In such an unfortunate eventuality, you will incur more severe civil liabilities.