What was the significance of the middle bolt?
Rashi, Rashbam #1 and R. Bachye #1: The middle bolt (which comprised three bolts), and which ran inside the wall from one end of each side to the other, measured thirty Amos inside the north and south walls, and twelve Amos inside the west wall. 1
Rashbam #2 (citing Shabbos, 98b) R. Bachye #2 and Targum Yonasan: The middle bolt ? miraculously 2 - went (through the planks) round all three sides 3 of the Mishkan. 4
Rashbam: Thereby joining the west wall with the north and south walls.
Targum Yonasan: It turned the corner - like a snake, whilst being inserted and straightened itself - like a stick, whilst being withdrawn .
Rashbam: The north, the west and the south. This constituted a miracle, as the Gemara states in Shabbos, 98b.
R. Bachye: In which case it was seventy Amos long. See R. Bachye, who cites Tosfos as the author of this interpetation of the Gemara in Shabbos.
Assuming the B'ri'ach ha'Tichon was seventy Amos long, from where did Yisrael obtain it?
R. Bachye (citing the Tanchuma): Ya'akov Avinu (who was the 'Kav ha'Emtza'i' - the Midah of Tif'eres) 1 brought it down to Egypt and instructed his children that, when they left Egypt, they should take it with them.
Targum Yonasan: Angels brought from the trees that Avraham Avinu planted in Be'er-Sheva, and deposited in the Yam-Suf at Keri'as Yam-Suf, and when Yisrael crossed the Yam-Suf, they took it and used it for the middle bolt of the Mishkan. 2
See R. Bachye.
In keeping with the Pasuk in Tehilim (89:3) 'The world is built on Chesed' - the Midah that prompted Avraham to plant the trees for the benefit of others.
