Section 81. The sum of eighteen thousand dollars may be paid annually from the state treasury to furnish relief to firemen injured in the performance of their duty at a fire or in going thereto or returning therefrom, or while engaged in company drills, when such drills are ordered by the chief, acting chief or board of engineers of the fire department, or required by city ordinance or town by-law, and to widows and children of firemen killed in the performance of such duty. Payments on account of said relief shall be determined in manner and amount, on properly approved vouchers, by the commissioners on firemen's relief, in the same manner as other claims against the commonwealth. Five hundred dollars of the amount hereby authorized may be expended for the traveling and incidental expenses of said commissioners.