Whenever twelve resident taxpayers in any township or townships petition the county commission for a bridge upon any public road in such township or townships, giving the name of the stream and the locality of the contemplated bridge, and the citizens of the township or townships raise one-half the estimated cost of such bridge and deposit such amount with the county treasurer and certify the fact to the county commission, the commission shall immediately make an appropriation for the other half; provided, sufficient funds are on hand in the county treasury or will be available out of the taxes collected for such year and not previously set apart for other purposes.