As already pointed out, the use of separate circuits for each sub-station, and for lighting and power loads at each sub-station in very long transmission systems, is often impracticable. Even in comparatively short transmissions the multiplication of circuits and the use of rather small and mechanically weak conductors increased the first cost of installation and the subsequent expense of inspection and repairs. An objection to operation with a single circuit in a transmission line that supplies widely separated sub-stations with lighting, power, and railway loads is the consequent difficulty of pressure regulation on the distribution lines at each sub-station. Such a transmission line necessarily delivers energy at different and fluctuating voltages at the several sub-stations, and these fluctuations are of course reproduced on the secondary side of the step-down transformers. Fortunately, however, the use of synchronous motor generators, either in place of or in connection with static transformers, goes far to solve the problem of pressure regulation for distribution circuits supplied with energy from transmission lines. This is due to the well-known fact that with constant frequency the speed of rotation for a synchronous motor is constant without regard to fluctuations in the applied voltage or changes in its load. With a constant speed at the motor and its connected generator it is of course easy to deliver current at constant voltage to the distribution lines. This constancy of speed makes the synchronous motor generator a favorite in large transmission systems with both power and lighting loads. The satisfactory lighting service in Buffalo, operated with energy transmitted from Niagara Falls, seems to be due in some measure to the use of synchronous motor generators at the sub-station in Buffalo, whence lighting circuits are supplied. As above stated, the three circuits that make up the transmission line between Niagara Falls and Buffalo are operated in multiple, and in the latter place there is a large load of both railway and stationary motors. As the three circuits are operated in multiple, they of course amount to only a single circuit so far as fluctuations of voltage due to changes in these several sorts of loads are concerned. According to vol. xviii., A. I. E. E., pages 125 and following, the load on the transmission system at Buffalo in 1901 was made up of about 7,000 horse-power in railway motors, 4,000 horse-power in induction motors, and 4,000 horse-power divided up between series arc lamps, constant pressure incandescent lamps, and continuous current motors. The railway load is operated through step-down transformers and rotary converters. The induction motors are connected either to the 2,000-volt secondary circuits of the step-down transformers or to service transformers supplied by these circuits. On these railway and stationary motor loads there is of course no necessity for close pressure regulation. Series arc lamps are operated through step-down transformers and synchronous motors direct-connected to constant continuous current dynamos. Continuous current stationary motors draw power from the transmission lines through step-down transformers and rotary converters, like the railway load. For the 2,200 volt circuits that supply service transformers for commercial arc and incandescent lighting the transmitted energy passes through step-down transformers and synchronous motor-generators. These motor-generators raise the frequency from twenty-five to sixty cycles per second. Finally the continuous current three-wire system for incandescent lighting at about 250 volts between outside wires is operated through step-down transformers and synchronous motors direct-connected to continuous current generators. For this last-named service rotary converters were at first tried, but were found to be impracticable because voltage fluctuations on the transmission line (due largely to the railway and motor loads) were reproduced on the continuous-current circuits by the rotary converters. Since the adoption of motor-generators this fluctuation of the service voltage is no longer present.