A wheel in the middle of (RV 'within') a wheel] an obscure expression. One explanation is that every wheel had another joined to it at right angles, so that the compound wheel would appear thus

from above, and could revolve backwards and forwards on one rim, and from side to side on the other. Another theory places the four wheels symmetrically thus

, in which arrangement, taking anyside as the front, the back wheel would overlap the front one and could be seen through the spokes of the latter. A third suggestion is that every wheel was made up of two concentric circles, the outer rim having a smaller wheel within it, surrounding the axle, thus

.

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