From and after the passage of this act, the commissioners' board of the respective counties of this Commonwealth may, in its discretion, pay out of the county funds not otherwise appropriated, and upon proper vouchers being given, a sum not exceeding one thousand dollars, annually, to the historical society of said county, to assist in paying the running expenses thereof. Where such a society is comprised of residents of more than one county, the commissioners of said respective counties may jointly pay said sum in such proportion as they shall agree.