מדרש על בבא קמא 121:7

"If fire go out and it find thorns": "thorns" are mentioned only to assign a limit (for his liability). If thorns are present (and carry the fire), there is a limit (for his liability). If they are not present (and he lights it directly) there is no limit. From here they ruled: If it crossed a river or a (public) way or a fence ten cubits high and caused damage, he is not liable. How is he perceived? He is perceived as standing in the midst of a beth-kor and damaging, (liability obtaining up to about 137 cubits on all sides.) R. Eliezer says: Sixteen cubits, as (the distance of) the public way. R. Akiva says: Fifty cubits. R. Shimon says: "Pay shall he pay, he that lights the fire" — all according to the fire (i.e., according to the height of the fire and its mass. The bigger it is, the farther it travels.) It once happened that a fire crossed the Jordan and caused damage because of its mass. When is this so? When it moves in spurts; but when it moves contiguously, even up to a mil, he is liable.