the theory of the collision of bodies was propounded by the royal society in 1668 for the consideration of mathematicians. wallis, christopher wren, and christian huygens sent correct and similar solutions, all depending on what is now called the conservation of momentum; but, while wren and huygens confined their theory to perfectly elastic bodies (elastic collision), wallis considered also imperfectly elastic bodies (inelastic collision). this was followed in 1669 by a work on statics (centres of gravity), and in 1670 by one on dynamics: these provide a convenient synopsis of what was then known on the subject.