In order to acquire by prescription the easements referred to in the preceding article, the time of possession shall be computed thus: in positive easements, from the day on which the owner of the dominant estate, or the person who may have made use of the easement, commenced to exercise it upon the servient estate; and in negative easements, from the day on which the owner of the dominant estate forbade, by an instrument acknowledged before a notary public, the owner of the servient estate, from executing an act which would be lawful without the easement. (538a)