I found this: "The Persian wheel, is a great improvement on the ‘mohte’ [a water lifting device using a leather bucket and a rope to pull up water usually by a draught animal], as its chain of buckets imposes an almost constant load on the drive shaft to the wheel. Persian wheels are usually driven by some form of right angle drive. The first is the most common, where the drive shaft from the secondary gear is buried and the animals walk over it; this has the advantage of keeping the Persian wheel as low as possible to minimize the head through which water is lifted. The second example is a traditional wooden Persian wheel mechanism where the animal passes under the horizontal shaft. The sweep of a Persian wheel carries an almost constant load and therefore the animal can establish a steady comfortable pace and needs little supervision.”
http://base.d-p-h.info/en/fiches/dph/fiche-dph-7866.html