Provided always, that every such lease for two lives shall contain a clause obliging the tenant or tenants to fence and inclose in a sufficient and lasting manner all the lands so leased within the space of thirty years, and two third parts thereof within the space of twenty years, and one third part thereof within the space ten years, if the said lease shall continue for such respective terms; and that every such lease for any term of years exceeding nineteen years, shall contain a clause, obliging the tenant or tenants to fence and inclose in like manner all the lands so leased during the continuance of such term, and two third parts thereof before the expiration of two third parts of such term, and one third part thereof before the expiration of one third part of such term.