In 1872, the former governor of California, Leland Stanford, a businessman and race-horse owner, hired Muybridge for some photographic studies. He had taken a position on a popularly debated question of the day — whether all four feet of a horse were off the ground at the same time while trotting. Stanford sought out Muybridge and hired him to settle the question. In 1872, Muybridge settled Stanford's question with a single photographic negative showing his Standardbred trotting horse named Occident, airborne at the trot.
Stanford also wanted a study of the horse at a gallop. Muybridge planned to take a series of photographs on 15 June 1878, at Stanford's Palo Alto Stock Farm. He placed numerous large glass-plate cameras in a line along the edge of the track; the shutter of each was triggered by a thread as the horse passed He copied the images in the form of silhouettes onto a disc to be viewed in a machine he had invented, which he called a "zoopraxiscope". This device was later regarded as an early movie projector, and the process as an intermediate stage toward motion pictures or cinematography.
The study is called “Sallie Gardner at a Gallop” or “The Horse in Motion;” it shows images of the horse with all feet off the ground.