 Complete the following phrase:  Anomalous sound propagation in the atmosphere can occur in certain conditions of wind, temperature and pressure. Such conditions enable sound to travel in refraction channels over long distances until returning to the earths surface, and it thus may not be heard in intervening locations. As one website refers to it, an acoustic shadow is to sound what a mirage is to light. For example, at the Battle of Iuka, a northerly wind prevented General Ulysses S. Grant from hearing the sounds of battle and sending more troops. Many other instances of acoustic shadowing were prevalent during the American Civil War, including the Battles of Seven Pines, 