At the expiration of present contracts for official advertising in cities of the second class A, and every three years thereafter, the mayor of any said city shall contract for a term of three years with not less than four nor more than five newspapers of general circulation, one of which may be a newspaper published weekly on Sundays and the others daily newspapers published within the county in which such city of the second class A is situated, for the publication of all official advertising, the cost of which is payable out of the city treasury; which said official advertising shall include all ordinances of councils, mayors' proclamations, all official reports of the city officers, all notices for the opening, widening, straightening, grading, paving, and curbing and vacation of streets, lanes, and alleys, and the construction of sewers, including all viewers' reports and proposals for public work and supplies; such contracts to be let to the lowest bidder, agate measure per line for each thousand of circulation; the basis of the circulation to be the average daily sales (exclusive of Sunday, weekly, semi-weekly, and tri-weekly issues, and also exclusive of all credits for returned or unsold newspapers and newspapers distributed free of charge) of such daily newspaper and the average weekly sales (exclusive of any semi-weekly and tri-weekly issues, and also exclusive of all credits for returned or unsold newspapers and newspapers distributed free of charge) of such Sunday newspaper for the period of six months next preceding the month in which such advertising may be published.