In order for a common carrier to avail himself of the act of God exception under Code Section 46-9-1 or the contractual exception under Code Section 46-9-2, he must establish not only that the act of God or the contractually excepted fact ultimately occasioned the loss but that his own negligence did not contribute thereto. (Civil Code 1895, § 2265; Civil Code 1910, § 2713; Code 1933, § 18-103.)