What is the test for a lender to charge for the use of a borrower's horse?

Lord Holt, C.J., in Coggs v. Bernard, 2 Ld. Raym. 909, at 915, lays down the law as follows: As to the second sort of bailment, viz., commodatum or lending gratis, the borrower is bound to the strictest care and diligence, to keep the goods, so as to restore them back again to the lender, because the bailee has a benefit by the use of them, so as if the bailee be guilty of the least neglect, he will be answerable; as if a man should lend another a horse, to go westward, or for a month; if the bailee go northward, or keep the horse above a month, if any accident happen to the horse in the northern journey, or after the expiration of the month, the bailee will be chargeable; because he has made use of the horse contrary to the trust he was lent to him under, and it may be if the horse had been used not otherwise than he was lent, that accident would not have befallen him.