The ways that bodies of water can combine is important for a number of reasons. For example, it may allow two pits of rain water to combine together to make the minimum quantity for a mikveh. Also we have learnt that if a valid mikveh is considered connected to a pit of drawn water it can turn that pit into a valid mikveh. Ordinarily, the radius of a hole or pipe that connects the two mikvaot must be k’shfoferet ha’nod (“like the spout of a leather water bottle”) – the width of two fingers. The Mishnah that we will focus on however discusses whether a crack, not a hole, in the wall that divides two bodies of water is enough for them to be considered joined.
The first opinion rules that only if the cracks are in the direction of the sheti would the pits be considered joined. R’ Yehuda however rules in the opposite way that the crack must be in the direction of the erev. What exactly is the sheti and erev for a wall is the subject of debate in the mefarshim.
The Rambam and Bartenura explain that sheti refers to a vertical crack while erev refers to a horizontal one. The Rash and Rosh however hold that reverse is true. The Mishnah Achrona lends support to the first opinion as the Rash himself cites a Tosefta (see Ohalot 6:3) referring to the sheti as being in the vertical direction. The Tifferet Yisrael suggests that the latter opinion may be borrowing these terms from fabric on a weaving machine, where the sheti refers to the long warp threads. This would match the generally larger dimension of the wall – its width.
Continuing with the understanding of the Rambam, how should we understand the debate? What difference does the direction of the crack make? The Tifferet Yisrael explains that there are two ways to understand what the appearance of a crack does. The first is that there are two separate laws relating to cracks and holes. Holes require a particular diameter or area while cracks do not. This is provided that the crack is substantial and lasts. Consequently according to the first opinion a vertical crack would combine the mikvaot. In the presence of a horizontal crack the wall is effectively divided in two, with the top piece resting heavily on the lower one and it is likely to close. R’ Yehuda however understands that cracks are no different to holes and require a gap. However in the case of a horizontal gap, since the structure of the upper half is unstable it is likely to become comprised and is therefore considered not present. This is consistent with R’ Yehuda’s opinion earlier regarding a comprised wall (6:1).
The Tifferet Yisrael however anticipates the following question. According to his explanation the Rambam’s ruling are difficult. The Rambam rules like R’ Yehuda earlier that a rickety wall does not required a breach k’shfoferet ha’nod (Mikvaot 5:4), yet rules like the Chachamim here that the mikvaot only combine in the case of a vertical crack (Mikvaot 8:5). Consequently if R’ Yehuda’s opinion here is in line with the earlier Mishnah then the Rambam’s rulings are contradictory. Nevertheless the Tifferet Yisrael explains that the case here is different. In the earlier case the wall already could not stand independently. In that case the Rambam agrees with R’ Yehuda. Here however, the wall has only a crack and will eventually become weakened which is not reason enough for the Rambam.
The Mishnah Achrona explains the opinion of R’ Yehuda in a different manner, thereby avoiding the problem dealt with by the Tifferet Yisrael. He explains that R’ Yehuda requires the area of crack to be k’shfoferet ha’nod (rather than ordinarily requiring the diameter to be so). Moreover the crack must specifically be a horizontal one such that waters combine across the full width. The reason is that wherever anyone immerses, they will be opposite the crack that combines the two mikvaot (see also the Taz on YD 101:54). (The Tifferet Yisrael however finds this explanation difficult since even with a horizontal crack, one could immerse small utensils without them passing in front of that crack.)
The R’ Yerucham however explains that both the Chachamim and R’ Yehuda require the area of the (valid) cracks to be k’shfoferet ha’nod. If so, about what do they argue? The Mishnah Achrona explains that according to this understanding, everyone would agree that the direction of the crack does not matter. They are however concerned that if the water level changes, thereby reducing the water flow through the crack, one will not notice and immerse in an invalid mikveh. The Chachamim argue that this would not be a concern in the case of a vertical crack as the changes to the water level are visible. R’ Yehuda however argues that cracks are generally fine and gradual changes would not be as recognisable compared with horizontal crack that completely appears above the water line.
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