Variances. Variances may be granted by the Board of Appeals with respect to particular land or structures from the terms of the applicable Zoning Bylaw when a particular use or dimensional variance is sought and when it specifically finds that, owing to circumstances relating to the soil condition, shape, or topography of such land or structures and especially affecting such land or structures but not affecting generally the zoning district in which it is located, a literal enforcement of the provisions of the ordinance or bylaw would involve substantial hardship, financial or otherwise, to the petitioner or appellant and that desirable relief may be granted without substantial detriment to the public good and without nullifying or substantially derogating from the intent or purpose of such ordinance or bylaw. Variances properly granted prior to January 1, 1976, but limited in time may be extended on such terms and conditions that were in effect for such variances upon said effective date.