The City Council may borrow money on the credit of the City and issue the bonds of the City therefor, for any purpose within the scope of its powers; provided that the net bonded indebtedness incurred for all public purposes shall not at any time exceed ten percentum of the assessed valuation of all real and personal property in the City; provided, further, that in case of fire, flood or other calamity the legislative body may borrow for the relief of the inhabitants of the City and for the preservation of Municipal property, a sum not to exceed three-eighths of one percentum of the assessed valuation of all real and personal property in the City, due in not more than five years, even if such loan would cause the indebtedness of the City to exceed the said ten percentum limitation; and provided, further, that in computing the net bonded indebtedness for the purposes hereof there shall not be included bonds issued in anticipation of the payment of special assessments even though they are also a general obligation of the City, mortgage bonds which are secured only by a mortgage on the property or franchise of a public utility and any other bonds which are excluded therefrom by law; and the resources of any debt retirement or sinking fund pledged for the retirement of any outstanding bonds which are included within said ten percentum limitation, shall be deducted from the amount of such bonded indebtedness.