If in the agreement provision be made for a method of naming or appointing an arbitrator or arbitrators or an umpire, such method shall be followed; but if no method be provided therein, or if a method be provided and any party thereto shall fail to avail himself of such method, or if for any other reason there shall be a lapse in the naming of an arbitrator or arbitrators or umpire, or in filling a vacancy, then upon the application of either party to the controversy the court shall designate and appoint an arbitrator or arbitrators or umpire, as the case may require, who shall act under the said agreement with the same force and effect as if he or they had been specifically named therein; and unless otherwise provided in the agreement the arbitration shall be by a single arbitrator.