The board of assessors shall have the power to administer oaths or affirmations for all purposes necessary in the discharge of their duties. The assessment as aforesaid, when finally made and determined, shall remain the lawful assessment for purposes of city taxation for such city until the next triennial assessment to be made by such board of assessors as herein provided, subject, however, to such additions and transfers, and other incidental alterations as such board of assessors may lawfully make to perfect the assessment for each year's tax levy.
53 P.S. § 25898