A wide variety of systems have been employed to meet these conditions. There is the travelling automobile station, in which the installation is mounted upon a motor-car. In this instance the whole equipment is carried upon a single vehicle, while the antenna is stowed upon the roof and can be raised or lowered within a few seconds. If motor traction is unavailable, then animal haulage may be employed, but in this instance the installation is divided between two vehicles, one carrying the transmitting and receiving apparatus and the generating plant, the other the fuel supplies and the aerial, together with spare parts. The motive power is supplied by a small air cooled petrol or gasoline motor developing eight horse-power, and coupled direct to a 2-kilo watt alternator. At one end of the shaft of the latter the disk discharger is mounted, its function being to break up the train of waves into groups of waves, so as to impart a musical sound to the note produced in the receiver. A flexible cable transmits the electric current from the generator to the wagon containing the instruments. The aerial is built up of masts carried in sections. The Germans employ a mobile apparatus which is very similar, but in this instance the mast is telescopic. When closed it occupies but little space. By turning the winch handle the mast is extended, and can be carried to any height up to a maximum of about 100 feet. The capacity of these mobile stations varies within wide limits, the range of the largest and most powerful installations being about 200 miles. The disadvantage of these systems, however, is that they are condemned to territories where the ground at the utmost is gently undulating, and where there are roads on which four-wheeled vehicles can travel. For operation in hilly districts, where only trails are to be found, the Marconi Company, has perfected what may be described as "pack" and "knapsack" installations respectively. In the first named the whole of the installation is mounted upon the backs of four horses. The first carries the generator set, the second the transmitting instruments, the third the receiving equipment, and the fourth the detachable mast and stays.