Section 26. If the owner of a lot abutting upon a public way in a city dedicates to the public or permits it to use a portion of his land lying between said way and a building fronting on the same, he shall keep such portion in such condition as, in the opinion of the aldermen, the safety and convenience of the public requires; and if he, after reasonable notice given by the aldermen or the mayor, neglects or refuses to make the specific repairs or improvements so required, or to close the same to public use by a substantial railing or guard, the aldermen may put said land into such condition and assess the expense thereof upon the owner; and assessments so made may be collected in the same manner as betterment assessments.