On daf 15: the Gemara brings a braissa that shavas b'tel is considered ok because of the sides (using his asik?) In support of omed m'elav
On 25a r chisda says r sheshes would agree that mechitzos would be considered significant for those on top of the tel as an effective addition
How are we to understand this (is it a matter of visibility rather than halachic significance?) wouldn't that be true go all mechitza a"g mechitza
Moshe Rubin, Brooklyn, ny
1) The mechitzos on 25a are real mechitzos with genuine Halachic significance. There is a big difference between the Gemara 15a and the Gemara 25a. 0n 15a he does not live on the Tel. He was on a journey and it got dark on Friday night so he started Shabbos on the Tel. Because of Gud Asik he is mukaf; he is surrounded by a wall; but he is not Mukaf l'Dirah because he has no intention of surrounding the Tel in order to be a place to live since he will not live there. He is only there because of getting stuck there for one Shabbos. Rashi 15a DH veHoo writes that the Mechitzos there only help if the tel is less than bais sasayim because it is not mukaf l'dirah. On 15a he certainly may not carry on the Tel. Rather, the issue is whether he can walk anywhwere he likes on the Tel and also walk 2000 amos outside the Tel. On 15a the mechitzos are imaginary ones of Gud Asik.
2) On 25a Rav Chisda said that Rav Sheshes will agree with him that if he made a real mechitzah on the Tel that this helps and he can now carry on the Tel even though the Tel is bigger than beis sayasayim. This is because he really is living in the airspace of the new mechitzos. Rashi DH Ho'il writes that it is only the new mechitzos which are now helping him to live on top of the tel; the old ones are not helping at all. "Mechitzah Al Gabei Mechitzah" only invalidates the second Mechitzah if the second Mechitzah does not add on anything to the first; whilst in Rav Chisda's case the new mechitzos become the effective ones.
KOL TUV
Dovid Bloom