414. Obligation not to derogate from grant.

In addition to any obligation to give quiet enjoyment, the landlord is under an obligation not to derogate from his grant; and this obligation is not excluded by an express covenant for quiet enjoyment1. Thus, where a lease is made for a particular purpose, the landlord is under an obligation not to use adjoining land retained by him in such a way as to render the demised premises unfit or materially less fit for that purpose2. For the court to give relief on this ground the interference must be substantial3. User of adjoining land which merely makes the user