Shabbat 5B

היה קורא בספר על האיסקופה ונתגלגל הספר מידו ⁠— גוללו אצלו. היה קורא בראש הגג ונתגלגל הספר מידו, עד שלא הגיע לעשרה טפחים ⁠— גוללו אצלו. משהגיע לעשרה טפחים ⁠— הופכו על הכתב. והוינן בה: אמאי הופכו על הכתב? הא לא נח!

One who was reading a sacred book in scroll form on Shabbat on an elevated, wide threshold and the book rolled from his hand outside and into the public domain, he may roll it back to himself, since one of its ends is still in his hand. However, if he was reading on top the roof, which is a full-fledged private domain, and the book rolled from his hand, as long as the edge of the book did not reach 10 handbreadths above the public domain, the book is still in its own area, and he may roll it back to himself. However, once the book has reached within 10 handbreadths above the public domain, he is prohibited to roll it back to himself. In that case, he may only turn it over onto the side with writing, so that the writing of the book should face down and should not be exposed and degraded. And we discussed this halakha: Why must he turn it over onto the side with writing, and he is prohibited to bring the book back to himself? Didn’t the book not yet come to rest upon a defined area in the public domain? Even if he brought it back it would not constitute lifting.

ואמר רבא: בכותל משופע. אימור דאמר רבא בספר, דעביד דנייח, מים מי עבידי דנייחי?!

And Rava said: It is referring to the case of an inclined wall. Because it is inclined, the scroll is resting upon it to some degree. However, that answer is not effective in explaining the case of gathering water. Say that Rava said that the legal status of the slanted wall is different, specifically with regard to a book, as it is wont to come to rest upon an inclined wall. In contrast, is water wont to come to rest upon an inclined wall? It continues flowing. Consequently, the question with regard to water remains.