Variances. To grant upon appeal or upon petition in cases where a particular use is sought with respect to a particular land or structure a variance from the terms of this Zoning Bylaw where the Board of Appeals find that, owing to circumstances relating to soil conditions, shape, or topography of such land or structures and especially affecting such land or structures but not affecting generally the zoning district in which they are located, a literal enforcement of the provisions of this bylaw would involve substantial hardship, financial or otherwise, to the 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 this bylaw.