A term or condition in a contract of employment , either express or implied from the circumstances of the employment, by which the employ^ agrees that dangers of injury ordinarily or obviously incident to the discharge of his duty in the particular employment shall be at his own risk. Narramore v. Railway Co., 96 Fed. 301, 37 C. C. A. 499, 48 L. It. A. 68; i^aulkner v. Mining Co., 23 Utah, 437, 66 Pac. 799; Railroad Co. v. Touoey, 67 Ark. 209, 54 S. W. 577, 77 Am. St. Rep. 109; Bodie v. Railway Co., 61 S. C. 468, 39 S. E. 715; Martin v. Railroad Co., 118 Iowa, 148, 91 N. W. 1034, 59 LL R. A. 698, 96 Am. St. Rep. 371.