After1 the first edition [of Taharas HaMayim] had already been typeset, I received a memorandum recorded for his own use by R. Yaakov Landau [of blessed memory], the rav of Bnei Brak in Eretz Yisrael, a distinguished scholar and chassid who was privileged to be a close disciple of the Rebbe Rashab in Lubavitch, and later too in Rostov. These notes list the views of the Rebbe Rashab as expressed to him in Rostov in the years 5678-79 (1918-19), in connection with the specifications for the mikveh which was to be built for his own use in his home. This memorandum was [later] perused by his son, the revered R. Yosef Yitzchak of Lubavitch. It is reproduced hereunder in full, exactly as received.

Regarding the mikveh: The Rebbe [Rashab] desired that they should construct one mikveh over another, in the following manner:

1. The lower mikveh was to contain the rainwater (mei geshamim); the upper mikveh which was situated over the floor [which divides the two mikvaos] was to contain ordinary water. It was in the latter that one immersed. (The Rebbe decided that the rainwater mikveh should be below because he wanted to take into account the [more stringent] ruling of the Raavad in the halachic question of “adding a se’ah and removing a se’ah,” and when the rainwater mikveh is situated below the other it is most unlikely that the two bodies of water will mix, because of their differing temperatures.)2

2. The connection (the hashakah) between the two mikvaos is via a hole whose diameter is a tefach. (As to the reason for this, the Rebbe told me that at the time he did not recall it; however, there was a reason.) [Moreover,] two such holes should be made, at a distance from each other, simply for fear that one hole might be blocked at some time and not be noticed.

3. There should be no drainage hole in either the upper mikveh nor the lower one. The hole for the entry of the rainwater should be located in the upper mikveh chamber — higher than the water level, so that there will be no question of [invalidating the mikveh by] seepage (zechilah). Through the upper mikveh the rainwater reaches the lower one.

4. In the floor separating the two mikvaos there should be a marble slab, approximately one amah (“cubit”) square, which, when removed, enables a person to enter the lower mikveh in order to dry it whenever there is a need to change the rainwater. The marble slab should be entirely removed before the lower mikveh is filled with the rainwater that reaches it via the upper mikveh chamber. The lower mikveh should be filled until the rainwater entirely covers the dividing floor. (I queried this procedure at the time, and recall too that the Rebbe saw my point. In practice, however, my notes testify that the procedure followed was as described: the lower mikveh was filled until the rainwater covered the floor.) In the middle of this marble slab was the hole that connected the two mikvaos [as in para. 2, above].

5. The volume of rainwater in the lower mikveh was double the required minimum, so that if it was partly displaced by any accumulation of silt, at least the minimal volume of rainwater would always be sure to remain.

6. The length of the mikveh was to be (in Russian terms) no less than 32 viershoks (i.e., 1.424 meters), so that a person would be able to immerse while lying his full length, with his head over the lowest step. The Tzemach Tzedek had once told the Rebbitzin [Rivkah], mother of the Rebbe [Rashab], that this was the ideal posture for immersion.

7. [Not only did] the rainwater (lit., “the kosher water”) reach the lower mikveh by hamshachah, i.e., by flowing over a porous substance, but [even] the water for the upper mikveh, which came from the municipal mains supply, also entered it by similar means within the wall.

8. The tap for the entry of the rainwater was not to be in the mikveh, but at some distance from it, so that the rainwater would not enter the mikveh as a direct result of human effort. Instead, after the tap had been opened at some distance, the rainwater would first flow through a pipe made of an absorptive substance; i.e., it would reach the mikveh by means of hamshachah. In the wall of the upper mikveh there was also a tap, situated higher than the water level (see para. 3, above). This used to be opened before the other tap (which was at some distance), so that the water would not enter the mikveh as a direct result of human effort, as mentioned above.

9. It is always desirable to have a constant storage of rainwater ready for changing. Next to this storage, which is situated underground in the yard, there should be an empty chamber from which one can open the tap which is located in the dividing wall of the underground storage chamber. This water should flow through a pipe of lean, porous cement.

10. A pump and rubber hose were used for cleaning the mikveh.

11. Rubber stoppers are not to be used for the holes, because according to the Rebbe these can contract tumah, spiritual uncleanliness.

12. I therefore made a lathed wooden stopper, in the middle of which was fitted a wooden tap, and this stopper was inserted into the concrete construction. A wooden tap in a wooden surrounding holds tight.

13. The roof over which the rainwater passes should have nothing to impede its flow, and certainly no tin gutters which are stopped up. It is preferable that the downspout from the roof should be straight, without bends. The rainwater should then flow through an underground pipe made of porous cement.

14. When calculating the minimal capacity of a mikveh, the Rebbe used to reckon an amah as 14 viershoks (i.e., 62.3 cm). The minimal capacity of a mikveh is thus 725.413,101 cc, i.e., approximately 726 liters.

15. The tap from the rainwater storage is of course situated a little higher than its floor level, so that any sediment which has entered with the rainwater from the roof will settle. Right at its floor level there is another opening to allow the water to be drained when the storage chamber needs to be cleaned.