Section 1. the Board of Street Commissioners of the City of Boston

may at any time within two years after the passage of this act abate such proportion of any assessment for a betterment made on account of the laying out and construction of Columbia Road from Franklin Park, in Dorchester, to Marine Park, in South Boston, not exceeding in the case of lots unimproved at the time of the passage of the order for the said laying out and construction twenty per cent, and in the case of lots improved at the time of the passage of said order and lots bordering on the parts of the said road formerly known as the Strandway and Dorchesterway thirty-three and one-third per cent, as said board shall deem just and expedient.