The power to regulate commerce, vested in congress, is the power to prescribe the rules by which it shall be governed, that is, the conditions upon which it shall be conducted, to determine when it shall be free, and when subject to duties or other exactions. The power also embraces within its control all the instrumentalities by which that commerce may be carried on, and the means by which it may be aided and encouraged. Gloucester Ferry Co. v. Pennsylvania, 114 U. S. 196, 5 Sup. Ct. 826, 29 L. Ed. 158. And see Gibbons v. Ogden, 9 Wheat. 227, 6 L. Ed. 23; Gilman v. Philadelphia, 3 Wall. 724, 18 L. Ed. 90; Welton v. Missouri, 91 U. S. 279, 23 L. Ed. 347; Leisy v. Hardin, 135 U. S. 100. 10 Sup. Ct. 081, 34 L. Ed. 12S; Kavanaugh v. Southern R. Co., 120 Ga. 02, 47 S. E. 520.