Charles Welsh, Dictionary of National Biography… (1912)
“ In 1872, whilst engaged in his official duties on the Pacific coast, he was consulted as to an old controversy in regard to animal locomotion, viz. whether a trotting horse at any portion of its stride has all its feet entirely off the ground. On the race-course at Sacramento, California, in May 1872, he made several negatives of Occident, a celebrated horse, while trotting laterally in front of his camera at speeds varying from 2 mins. 25 secs, to 2 mins. 18 secs, per mile. These experiments showed that the horse's four feet were at times all off the ground. ”
