What are these rotating beacons.??

The first nighttime airmail flights started on July 1, 1924. By eliminating the transfer of mail to rail cars at night, the coast to coast delivery time for airmail was reduced by two business days. Eventually, there were 284 beacons in service.^[3] With a June 1925 deadline, the 2,665 mile lighted airway was completed from New York to San Francisco. In 1927, the lighted airway was complete between New York and Salt Lake City, Los Angeles to Las Vegas, Los Angeles to San Francisco, New York to Atlanta, and Chicago to Dallas, 4121 miles in total. In 1933, the Transcontinental Airway System totaled 1500 beacons, and 18000 miles^[4]