When an increase of the indebtedness of the city, for a specific purpose, has been duly authorized in the manner required by law, the indebtedness so authorized, may first be incurred by the city by issuing, from time to time, as funds for the specific purpose are required, short term notes as hereinbefore provided; and the notes, so issued, shall thereafter be paid and funded out of the proceeds of bonds issued and sold for the authorized purpose.