If in either of the Cases aforesaid the said Arbitrators shall refuse, or shall for Seven Days after Request of either Party to such Arbitration neglect to appoint an Umpire, it shall be lawful for the Lord Ordinary, on the Application of either Party to such Arbitration, to appoint an Umpire ; and the Decision of such Umpire on the Matters on which the Arbitrators shall differ shall be final.