The Kiyor was a container of water used by the Kohanim to wash their hands and feet prior to beginning the avodah. It was a sanctified vessel, and if water would remain inside the Kiyor overnight it would become unfit for use in the Beis Hamikdash. This would have required the Kohanim to empty the Kiyor each night and refill it the next day, which is both degrading to the sanctified water of the Kiyor and a time-consuming task.To avoid this, they took advantage of the fact that if the water in the Kiyor were to be connected to a source of flowing water its sanctity would be nullified and thus would not become unfit for use if left out overnight. To this end they dug a pit in the floor of the Azarah and directed a stream of water through it (very likely the same water that flowed from the Eitam Spring). The Kiyor was lowered into this pit each night where it remained submerged in, and thus connected to, the flowing water.
A Kohen Gadol named Ben Katin, who lived during the Second Beis Hamikdash era, built a device called a Muchni [comparable to the English word machine] that was used to raise and lower the Kiyor out of the pit of water each day Although not described in detail, we do know that it was a free-standing structure, made of wood, and consisted of a rope and pulley system with a ratchet or gear purposely designed to generate a lot of noise as it operated. The Muchni allowed the Kiyor to be raised by just one person, quite a feat of engineering considering that the Kiyor, when full of water, weighed over 2½ tons!
The Kiyor was placed between the Ulam and the Mizbeyach, slightly off to the south (across from the ramp). Since the steps of the Ulam extended all the way across the face of the building, the Muchni and the pit that housed the Kiyor at night were built, out of necessity, directly upon these steps.
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