Ancient Beacon Towers
Ancient Chinese armies used fire and smoke signals to transmit military signals over long distances. These beacon towers were usually placed on strategically located yet difficult-to-reach locations, such as the Great Wall.
The towers were meant as a warning system against invaders. If invading enemy forces were spotted during daylight hours, wolf manure was burned. Why? Because its smoke would rise to extremely high altitudes without dispersing, and would be visible from hundreds of miles away: It was thus known as "wolf smoke."
If an approaching enemy was spotted at night, a succession of beacon towers would be lit to quickly relay the information back so that preparations for battle could be made.