The said several passenger street railway companies shall pay to the Mayor and City Council of Baltimore, a tax upon their gross receipts of nine percent, in quarterly installments, on the first day of January, April, July and October, in each year. P.L.L., 1888, §769 1915, §797 1927, §797 1930, §797 1938, §982 1949, §609 1969, §18-6 1882, ch. 229 1898, ch. 123, §797