To hear and decide, upon appeal or petition, requests for variances after public hearing for which notice has been given by publication and posting as provided in MGL c. 40A, § 11, and by mailing to all parties in interest. Variances from the term of this bylaw with respect to particular land or structures shall be granted only when the Board of Zoning Appeals specifically finds that, owing to circumstances relating to the 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 it is located, a literal enforcement of the provisions of this bylaw would involve substantial hardship, financial or otherwise, to the petitioner or applicant, or 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. No variance may authorize a use or activity not otherwise permitted in the district in which the land or structure is located; provided, however, that such variances properly granted prior to January 1, 1976, but limited in time, may be extended on the same terms and conditions there were in effect for such variance upon said effective date.