The long settled doctrine is that ordinarily where a person engages a contractor, who conducts an independent business by means of his own employees, to do work not in itself a nuisance (as cases put it), he is not liable for the negligent acts of the contractor in the performance of the contract. Terranella v. Union Bldg. & Construction Co., 3 N.J. 443, 446, 447 (1950); Mann v. Max, 93 N.J.L. 191, 193, 21 A.L.R. 1227 (E. & A. 1919); Cuff, Adm'x., v. Newark & New York R.R. Co., 35 N.J.L. 17 (Sup. Ct. 1870), affirmed 35 N.J.L. 574 (E. & A. 1871). Certain exceptions have come to be accepted, i.e., (a) where the landowner retains control of the manner and means of the doing of the work which is the subject of the contract; (b) where he engages an incompetent contractor; or (c) where, as noted in the statement of the general rule, the activity contracted for constitutes a nuisance per se. Terranella v. Union Bldg. & Construction Co., supra; Bergquist v. Penterman, 46 N.J. Super. 74 (App. Div. 1957), certification denied 25 N.J. 55 (1957); Trecartin v. Mahony-Troast Construction Co., 18 N.J. Super. 380 ...