YT: In 1878 Eadweard Muybridge photographed a horse named "Occident" in fast motion using a series of 12 stereoscopic cameras. The first experience successfully took place on June 11 at the Palo Alto farm in California. The cameras were arranged along a track parallel to the horse's, and each of the camera shutters was controlled by a trip wire which was triggered by the horse's hooves.

This is a follow up work to his expriment, proving that gallopping horses at times do not touch the ground at all.

From Wikipedia:
In 1872, former Governor of California Leland Stanford, a businessman and race-horse owner, had taken a position on a popularly-debated question of the day: whether all four of a horse's hooves left the ground at the same time during a gallop. Stanford sided with this assertion, called "unsupported transit", and took it upon himself to prove it scientifically. (Though legend also includes a wager of up to $25,000, there is no evidence of this.) Stanford sought out Muybridge and hired him to settle the question.[1] Muybridge's relationship with Stanford was long and fraught, heralding both his entrance and exit from the history books.
[...]
In 1877, Muybridge settled Stanford's question with a single photographic negative showing Stanford's racehorse Occident airborne in the midst of a gallop. This negative was lost, but it survives through woodcuts made at the time.