Surfactant prevents this collapse by reducing the surface tension of the fluids that line the lungs and helping to equalize the pressures between large and small air spaces. Surface tension is a measure of the attraction molecules of a fluid have for each other.
Buffalo Bill Cody, who later became famous for his Wild West Show, was a rider for the Pony Express and wrote of his experiences. We join Bill's story as he is hired - at the age of 15 - to ride a section of the trail that lies in modern-day Wyoming:
The Pony Express averaged 9 to 10 miles per hour over 15- to 25-mile stages, changing horses between stages. Many very long-distance rides, called endurance rides, have been completed on horses, in which the horse has to cover 100 miles in less than 24 hours. In the Middle East 26-mile races on horseback can be completed in around an hour.