The employer is bound to exercise ordinary care in the selection of employees and not to retain them after knowledge of incompetency; he shall use like care in furnishing machinery equal in kind to that in general use and reasonably safe for all persons who operate it with ordinary care and diligence. If there are latent defects in machinery or dangers incident to an employment, which defects or dangers the employer knows or ought to know but which are unknown to the employee, then the employer shall give the employee warning with respect thereto.