Section 70. There shall be a board of trustees of the Soldiers’ Home in Holyoke, consisting of seven persons, who shall be residents of the counties of Berkshire, Franklin, Hampden and Hampshire. Each of said counties shall be represented on said board by at least one trustee who is a resident therein. Upon the expiration of the term of office of a member, his successor shall be appointed by the governor, with the advice and consent of the council, to serve for seven years. The governor shall designate one of the members as chairman. The members shall serve without compensation, but shall receive their necessary expenses incurred in the discharge of their official duties.