Variances. The Board of Appeals may authorize, upon appeal or upon petition with respect to particular land or to structures thereon, a variance from the terms of this bylaw where, owing to circumstances relating to the soil conditions, shape or topography of such land or structures but not affecting generally the zoning district in which it is located, a literal enforcement of the provisions of this bylaw would involve substantial hardship, financial or otherwise, to the appellant, and where desirable relief may be granted without substantial detriment to the public good and without nullifying or substantially derogating from the intent or purpose of this bylaw, but not otherwise.