Perpetual overbalanced wheel with mercury, Siddhānta Śiromani

This perpetual wheel is a development of the one described in the 7th century by the Indian mathematician and astronomer Brahmagupta. Its movement is based on the displacement of mercury within the hollow spokes supporting the rim. Bhāskara modified the wheel giving the spokes a curved contour that recalls the shape of the flower Tabernaemontana coronaria. Because the spokes are designed as sections of a circle, mercury follows an asymmetrical path, curved downwards in the direction of descent of the wheel and curved upwards in the direction of ascent. In this way it was believed that the more sudden movement of mercury, due to the convex trajectory, brought a mechanical advantage in the phase of descent compared to the phase of ascent when, due to the concave shape of the path, mercury anticipated its movement to the centre. Combining a series of hollow spokes of this kind would have produced a series of pulses capable of keeping the wheel in motion. This model of a wheel was due to resurface in the Arabic engineering tradition and then in the Western tradition, where mercury would be replaced by mobile spheres.