1. The burglars have broken into this house twice. 2. Has the investigator examined the evidence yet? 3. The field-criminalist finished taking pictures ten minutes ago. 4. Have you ever seen a robbery? 5. Where were you at 10 o’clock yesterday? 6. The detective has already interviewed witnesses. 7. The robbers escaped in a stolen car yesterday. 8. We have just found footprints at the crime scene. 9. When did you interrogate the thief? 10. What have the thieves stolen from the safe? 11. Have the kidnappers called yet? 12. The beat officer hasn't walked the beat yet. 13. The mugger snatched the woman’s handbag when she was walking home. 14. Have you ever violated the law? 15. The police apprehended the arsonist last week.
If you have even been inside the cockpit of a plane, you will have noticed the large number of instruments on the walls and ceiling around the pilot’s seat. Many of these buttons, dials and screens are used in navigation. Unlike car drivers, pilots do not have the luxury of signs and arrows telling them where to go. Instead, they rely on sophisticated GPS systems. So how were pilots able to find their way in the days before radio and satellite communication?
In the USA, the problem of navigation first arose when the airmail postal service was introduced in 1911. In the beginning, huge bonfires were lit to show pilots where to land in the dark. Later, a more practical solution was found: the construction of giant concrete arrows on the ground. The arrows were 21 m long and painted bright yellow. To enable pilots to see them at night, a steel tower with lights was built in the middle. The top light turned around to attract the pilot’s attention, and two others lit up the arrow and flashed a code to identify its location.
The first of these arrows was laid in 1924, and by 1929, there were around 1,500 of them. They formed an illuminated path for airmail pilots stretching from New York to San Francisco. The arrows were spaced about 16 km apart and were an important part of the Transcontinental Airway System. But by the 1940s, new navigation techniques were being discovered and soon, the arrows and their towers were no longer needed. They were gradually abandoned and some of them were removed completely.
However, not all of the arrows have disappeared. In the state of Montana, they are still used for guiding pilots through the mountains. Some are still visible in the empty areas of other states such as Utah, Wyoming and Indiana. Yet either because of their size or due to their bad condition, the arrows are not easy to find. In many cases, if you want to see one, it is a question of knowing where to look.
Which is where retired couple Brian and Charlotte Smith can help. The two are devoted fans of the arrows and spend much of their time hunting them down. So far, they have located more than a hundred of them. Once they find an arrow, they map its location, photograph it and upload the details onto their website. Their aim is to preserve the memory of these historical structures and prevent them from being lost forever.
1. The burglars have broken into this house twice. 2. Has the investigator examined the evidence yet? 3. The field-criminalist finished taking pictures ten minutes ago. 4. Have you ever seen a robbery? 5. Where were you at 10 o’clock yesterday? 6. The detective has already interviewed witnesses. 7. The robbers escaped in a stolen car yesterday. 8. We have just found footprints at the crime scene. 9. When did you interrogate the thief? 10. What have the thieves stolen from the safe? 11. Have the kidnappers called yet? 12. The beat officer hasn't walked the beat yet. 13. The mugger snatched the woman’s handbag when she was walking home. 14. Have you ever violated the law? 15. The police apprehended the arsonist last week.
If you have even been inside the cockpit of a plane, you will have noticed the large number of instruments on the walls and ceiling around the pilot’s seat. Many of these buttons, dials and screens are used in navigation. Unlike car drivers, pilots do not have the luxury of signs and arrows telling them where to go. Instead, they rely on sophisticated GPS systems. So how were pilots able to find their way in the days before radio and satellite communication?
In the USA, the problem of navigation first arose when the airmail postal service was introduced in 1911. In the beginning, huge bonfires were lit to show pilots where to land in the dark. Later, a more practical solution was found: the construction of giant concrete arrows on the ground. The arrows were 21 m long and painted bright yellow. To enable pilots to see them at night, a steel tower with lights was built in the middle. The top light turned around to attract the pilot’s attention, and two others lit up the arrow and flashed a code to identify its location.
The first of these arrows was laid in 1924, and by 1929, there were around 1,500 of them. They formed an illuminated path for airmail pilots stretching from New York to San Francisco. The arrows were spaced about 16 km apart and were an important part of the Transcontinental Airway System. But by the 1940s, new navigation techniques were being discovered and soon, the arrows and their towers were no longer needed. They were gradually abandoned and some of them were removed completely.
However, not all of the arrows have disappeared. In the state of Montana, they are still used for guiding pilots through the mountains. Some are still visible in the empty areas of other states such as Utah, Wyoming and Indiana. Yet either because of their size or due to their bad condition, the arrows are not easy to find. In many cases, if you want to see one, it is a question of knowing where to look.
Which is where retired couple Brian and Charlotte Smith can help. The two are devoted fans of the arrows and spend much of their time hunting them down. So far, they have located more than a hundred of them. Once they find an arrow, they map its location, photograph it and upload the details onto their website. Their aim is to preserve the memory of these historical structures and prevent them from being lost forever.