Section 4. That it shall be the duty of the said commissioners, on the first Monday of September, in the year one thousand eight hundred and fifty-one, and on the same day in every third year thereafter, to certify the amount of the debt of the commonwealth held by them, to the governor, who shall direct the certificates representing the same to be cancelled, and on such cancellation issue his proclamation, stating the fact and the extinguishment and final discharge of so much of the principal of said debt.