Section 7. If an employer enters into a contract, written or verbal, with an independent contractor to do a part of such employer's work, or if such contractor enters into a contract with a sub-contractor to do all or any part of the work comprised in such contractor's work with the employer, such contract or sub-contract shall not bar the liability of the employer for injuries to the employees of such contractor or sub-contractor caused by any defect in the condition of the ways, works, machinery or plant, if they are the property of the employer or are furnished by him and if such defect arose, or had not been discovered or remedied, through the negligence of the employer or of some person intrusted by him with the duty of seeing that they were in proper condition.