When flying across the United States, you may see giant arrows made of concrete on the ground. Although noadays these arrows are basically places of curiousity, in the past, pilots absolutely needed them when flying from one side of the country to the other. ①The arrows were seen as being so successful that some people even suggested floating arrows on the Atlantic Ocean. ②Pilots used the arows as guides on the flights between New York and San Francisco. ③Every 16 kilometers, pilots would pass a 21-meter-long arrow that was painted bright yellow. ④A rotating light in the middle and one light at each end made the arrow visible at night. Since the 1940s, other navigation methods have been introduced and the arrows are generally not used today. Pilots flying through moountainous areas in Montana, however, do still rely on some of them.