airspeed in this example is about 10 times that of the wind speed, its 5% error can lead to great inaccuracies in wind speed calculations. Eliminating this error through the direct measurement of ground speed (instead of calculating it) was a major advance in the tachometric bombsights of the 1930s and 40s. Finally, consider errors of the same 5% in the equipment itself, that is, an error of 5% in the setting of the range angle, or a similar 5% error in the levelling of the aircraft or bombsight. For simplicity, consider that 5% to be a 5 degree angle. Using simple trigonometry, 5 degrees at 20,000 feet is approximately 1,750 feet, an error that would place the bombs far outside their lethal radius. In tests, accuracies of 3 to 4 degrees were considered standard, and angles as high as 15 degrees were not uncommon. Given the seriousness of the problem, systems for automatic levelling of bombsights was a major area of study before World War II, especially in the US. Early systems All of the calculations needed to predict the path of a bomb can be carried out by hand, with the aid of calculated tables of the bomb ballistics. However, the time to carry out these calculations is not trivial. Using visual sighting, the range at which the target is first sighted remains fixed, based on eyesight. As aircraft speeds increase, there is less time available after the initial spotting to carry out the calculations and correct the aircraft's flight path to bring it over the proper drop point. During the early stages of bombsight development, the problem was addressed by reducing the allowable engagement envelope, thereby reducing the need to calculate marginal effects. For instance, when dropped from very low altitudes, the effects of drag and wind during the fall will be so small that they can be ignored. In this case only the forward speed and altitude have any measurable effect. One of the earliest recorded examples of such a bombsight was built in 1911 by Lieutenant Riley E. Scott, of the U.S. Army Coast Artillery Corps. This was a simple device with inputs for airspeed and altitude which was hand-held while lying prone on the wing of the aircraft. After considerable testing, he was able to build a table of settings to use with these inputs. In testing at College Park, Maryland, Scott was able