William Hunting, The Art of Horse-Shoeing: A Manual for Farriers
“ This is more than doubtful, and I hold a very firm opinion that a plate of leather between the shoe and the foot has no such effect, whilst it interferes with the exactness of fit of the shoe. "Leathers" are useful on weak feet to protect a thin or defective sole from injury. When the under surface of a foot has been bruised, cut through, or when it is diseased, leather offers a useful protection, but when the sole is firm and sound it is quite unnecessary. ”
