Since the charter provision itself provides no method of formula for separating local and interstate revenue for the purpose of collecting the percentage sought to be exacted, the entire provision is void, even as to revenue from purely local sources. Damselle Howard v. Illinois Central R.R. Co., 207 U.S. 463, 28 S.Ct. 141, 52 L.Ed. 297; Weems v. Bruce (C.C.A. 10) 66 F.2d 304. And see Western Union T. Co. v. State of Kansas, 216 U.S. 1, 30 S.Ct. 190, 54 L.Ed. 355; Eureka Pipe Line Co. v. Hallanan, 257 U.S. 265, 42 S.Ct. 101, 66 L.Ed. 227.