“The principal is liable for his own negligence-or misconduct, and hence his liability rests on his own negligence or misconduct in the employment of his agents; and if he uses ordinary diligence in employing competent men it is enough to relieve him. He is not liable for the negligence of a fellow-servant while engaged in the same employment, unless he has been negligent in the selection of that servant, or retained him after knowledge of his incompetency. Nor will the fact that the person is found incompetent, of itself, and without more, show negligence of the master ; but it must further appear that the master knew or might have known, by ordinary diligence, the incompetency of the agent or servant. Law of master and servant, 333, 423-432; 2d Thomps., 969; Sherman & Red., 31; 1 Am. Rail., 536.”