Any body of water that is in contact with a mikveh, is considered as the mikveh itself and it can be used for immersion. When cavities that are next to the opening of a mikveh or the footprints left by animals share a point of contact with the water of the mikveh that is as large as the mouthpiece of a drinking pouch, they may be used for immersion.
Cavities at the side of the mikveh or crevices at the side of mikveh may be used for immersion even if they have only the slightest point of contact with the water of the mikveh.
When a needle was placed on the steps leading to a mikveh, one may move his hand back and forth in the water to create waves. Once the wave passes over the needle, it regains purity.
The following laws apply to a hidden reservoir in a mikveh: If the ground that separates between the mikveh and the reservoir is sturdy and capable of remaining over the course of time, one may not use the water in the reservoir for immersion unless it shares a point of contact with the mikveh as large as the mouthpiece of a drinking pouch. If it is not capable of remaining for an extended period, one may immerse in its water as long as it shares any contact whatsoever with the water of the mikveh.
🔗 Connection = Extension
Any body of water in contact with a valid mikveh is treated as part of that mikveh, including cavities at the opening, footprints of animals, and water in crevices. Even the slightest point of contact enables immersion in side cavities. A needle on the steps of a mikveh is purified by creating a wave that passes over it. A hidden reservoir separated by solid, durable earth is a separate body and cannot be used.
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Cracked Walls Between Mikveot and the Size of a Purifying Hole
When the wall between two mikveot is cracked in half, the two mikveot are considered as joined. If together they contain 40 se'ah, either of them may be used for immersion. If the wall is cracked horizontally, they are not considered as joined unless there is a point of contact the size of the mouthpiece of a drinking pouch. If the water is joined above the wall, they are considered as joined provided there is a layer of water even as thin as a garlic peel connecting them over the width of a space the size of the mouthpiece of a drinking pouch.
How large is the measure of a hole the size of the mouthpiece of a drinking pouch? A circle with a diameter the width sufficient to enable two average-sized fingers of an ordinary person to rotate within it. The fingers mentioned do not refer to the thumb, but to the first two of the four on the palm of one's hand. Any entity present in the hole the size of the mouthpiece of a drinking pouch reduces that measure. This applies even to entities that were created from the water. If there is a doubt whether a hole is the size of the mouthpiece of a drinking pouch or not, the mikveot are not considered as joined. The rationale is that the primary obligation to immerse is Scriptural. And whenever the primary obligation is Scriptural, even when the measure stated is conveyed by the Oral Tradition, we rule stringently when there is an unresolved doubt regarding its measure.
🧱 Walls and Holes
A wall between two mikveot cracked vertically (through its full height) joins them; a horizontal crack does not. The standard hole size for connection is a circle whose diameter allows two average fingers to rotate. This is the same measure used for chests in the sea and for purifying distant mikveot from each other.
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Purifying One Mikveh from Another and Wadi Cavities
Mikveot may be purified from each other, an upper mikveh from a lower one and a far-away mikveh from one that is close. What is implied? An earthenware or lead pipe - for pipes do not disqualify a mikveh - is brought and inserted into the acceptable mikveh. One places his hand under it until it becomes full and then takes it and connects it so that the water in the pipe will become intermingled with the water of the other mikveh. Even if the point where they become intermingled is only a hairsbreadth, it is sufficient. Then the two mikveot connected by the pipe are considered as one.
The following rules apply when there are three cavities in a wadi, the higher one and the lower one contain only 20 se'ah, the middle one contains 40 se'ah, and a current of rain water flows through the wadi. Although the current of water flows into the cavities and out of them, it does not join the cavities as one. Hence, only the middle one is acceptable for immersion. The rationale is that water that is flowing does not join mikveot unless it collects in one place.
🏔️ Upper and Lower Mikveot
A mikveh lacking 40 se'ah can be made valid by connecting it — even via a pipe — to another mikveh that already has 40 se'ah. An upper mikveh can purify a lower one and vice versa. In a wadi: if the middle of three cavities has 40 se'ah but a flowing current connects all three, the higher and lower cavities each become valid for immersion regardless of their individual volume.
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Mud, Worms, Sequential Immersion, and Pressing Straw
Soft mud from which a cow drinks may be measured as part of a mikveh. If it is so thick that a cow would not drink from it, it should not be measured as part of it.
When a mikveh contains 40 se'ah of water and mud, one may immerse in both the mud and the water. In which mud may one immerse? In soft mud over which water collects. If the water was on one side and the soft mud on the other side, one may immerse in the water, but not in the mud.
When a mikveh contains exactly 40 se'ah of water and two people descended and immersed, one after the other, the first is pure and the second remains impure. The rationale is that some of the water contained in the original 40 se'ah was removed. This stringency applies even in the feet of the first person were still in the water when the second immerses. If one immersed a thick sponge or the like in such a mikveh and lifted it up, as long as part of the sponge is touching the water, one who immerses afterwards is pure. The rationale is that all of the water is considered as intermingled. When one immerses a bed or the like, even though he pushed its legs into the thick mud on the bottom of the mikveh until the water covers it from above, it is pure. The rationale is that it does not become sunk in the water until it becomes immersed in the water first. When one immerses a large pot in a mikveh with a limited amount of water, it is impure, as it was beforehand, because the water will splash out of the mikveh. Thus the mikveh will be less than 40 se'ah. What should he do? He should lower it via its opening, turn it over in the mikveh, immerse it, and lift it up by its base, so that the water inside will not become "drawn," and return to the mikveh and disqualify it.
When the water in a mikveh is dispersed, one may press even bundles of straw and bundles of reeds into the water from the sides until the water level will rise and one can descend and immerse within.
🐛 Mud and Practical Scenarios
Mud soft enough that a cow would drink from it counts toward 40 se'ah; thicker mud does not. Red worms created from the water are acceptable for immersion. If exactly 40 se'ah exists and two people immerse sequentially, the first is pure but the second is not — some water clings to the first person when they exit. One may press bundles of straw into the mikveh from the sides to raise the water level.
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🎓 Key Principles
Chapter 8
🔗
Contact Creates a Single Body of Water Any water in physical contact with a valid mikveh — even through the slightest connection — shares the mikveh's status and may be used for immersion.
📐
Two Fingers Wide: The Standard Measure for Holes The hole size required to join two bodies of water for immersion purposes is a circle wide enough for two average-sized fingers to rotate within it.
🏔️
Mikveot Can Purify Each Other Through Piping A mikveh lacking 40 se'ah can borrow validity from a connected mikveh that has the required amount, even across distance via a non-vessel pipe.
⚖️
Sequential Immersion in an Exact Mikveh When a mikveh has exactly 40 se'ah, the first person to immerse is purified but the second is not — water clinging to the exiting body reduces the volume below the required minimum.