ground speed, which was multiplied by the time taken to hit the ground, and then a pointer in the sight was set to an angle looked up on a table. The bomb aimer then watched the target in the sight until it crossed the pointer, and dropped the bombs. Similar bombsights were developed in France and England, notably the Michelin and Central Flying School Number Seven bombsight. While useful, these sights required a time-consuming setup period while the movement was timed. A great upgrade to the basic concept was introduced by Harry Wimperis, better known for his later role in the development of radar in England. In 1916 he introduced the Drift Sight, which added a simple system for directly measuring the wind speed. The bomb aimer would first dial in the altitude and airspeed of the aircraft. Doing so rotated a metal bar on the right side of the bombsight so it pointed out from the fuselage. Prior to the bomb run, the bomber would fly at right angles to the bomb line, and the bomb aimer would look past the rod to watch the motion of objects on the ground. He would then adjust the wind speed setting until the motion was directly along the rod. This action measured the wind speed, and moved the sights to the proper angle to account for it, eliminating the need for separate calculations. A later modification was added to calculate the difference between true and indicated airspeed, which grows with altitude. This version was the Drift Sight Mk. 1A, introduced on the Handley Page O/400 heavy bomber. Variations on the design were common, like the US Estoppey bombsight. All of these bombsights shared the problem that they were unable to deal with wind in any direction other than along the path of travel. That made them effectively useless against moving targets, like submarines and ships. Unless the target just happened to be travelling directly in line with the wind, their motion would carry the bomber away from the wind line as they approached. Additionally, as anti-aircraft artillery grew more effective, they would often pre-sight their guns along the wind line of the targets they were protecting, knowing that attacks would come from those directions. A solution for attacking cross-wind was sorely needed. Vector bombsights Calculating the effects of an arbitrary wind on the path of an aircraft was already a well-understood