22. The dominant owner must exercise his right in the mode which is least onerous to the servient owner; and, when the exercise of an easement can without detriment to the dominant owner be confined to a determinate part of the servient heritage, such exercise shall, at the request of the servient owner, be so confined. Illustrations (a) A has a right of way over B's field. A must enter the way at either end and not at any intermediate point. (b) A has a right annexed to his house to cut thatching-grass in B's swamp. A, when exercise his easement, must cut the grass so that the plants may not be destroyed.