The draws of the St. Louis Southwestern railroad bridge, mile 98.9 at Clarendon, the Missouri Pacific railroad bridge, mile 196.3 at Augusta and the Missouri Pacific railroad bridge, mile 254.8 at Newport, shall open on signal if at least eight hours' notice is given. The draws of any of these bridges need not be opened for a vessel that arrives later than two hours after the time specified in the notice, unless a second notice of at least eight hours is given.