In this machine there are two articulated rhombuses pivoted together in a vertex Q.Two opposite vertices (different from Q) of each rhombus move in a straight path, and the two paths are intersecting in O. The two vertices P and R (each one on different rhombs) opposite to Q are corresponding in the isometry product of two axial symmetries. A pantograph of Sylvester is pivoted on O and connects the points P and R. Thus it is possible to give a mechanical proof that the product of two reflectionsis a rotation.