The Lap Engine is a beam engine designed by James Watt, built by Boulton and Watt in 1788. It is now preserved at the Science Museum, London. It is important as both an early example of a beam engine by Boulton and Watt, and also mainly as illustrating an important innovative step in their development for its ability to produce rotary motion. The engines name comes from its use in Matthew Boulton's Soho Manufactory, where it was used to drive a line of 43 polishing or lapping machines, used for the production of buttons and buckles.