It is obvious that the success of this method depends upon the maintaining of absolute uniformity in the motion of the two cylinders, and this is managed in a very ingenious way. It will be remembered that one method of securing uniformity in a number of sub-clocks under the control of a master-clock is that of adjusting the sub-clocks to go a little faster than the master-clock. Then, when the sub-clocks reach the hour, they are held back by electro-magnetic action until the master-clock arrives at the hour, when all proceed together. A similar method is employed for the cylinders. They are driven by electric motors, and the motor at the receiving end is adjusted so as to run very slightly faster than the motor at the sending end. The result is that the receiving cylinder completes one revolution a minute fraction of a second before the transmitting cylinder. It is then automatically held back until the sending cylinder completes its revolution, and then both commence the next revolution exactly together. The pause made by the receiving cylinder is of extremely short duration, but in order that there shall be no break in the spiral traced by light upon the film, the pause takes place at the point where the ends of the film come together. In actual practice certain other details of adjustment are required to ensure precision in working, but the main features of the process are as described. Although the above photo-telegraphic process is very satisfactory in working, it has been superseded to some extent by another process of a quite different nature. By copying the original photograph through a glass screen covered with a multitude of very fine parallel lines, a half-tone reproduction is made. This is formed of an immense number of light and dark lines of varying breadth, and it is printed in non-conducting ink on lead-foil, so that while the dark lines are bare foil, the light ones are covered with the ink. This half-tone is placed round a metal cylinder having the same movement as the cylinders in the previous processes, and a metal point, or “stylus” as it is called, is made to rest lightly upon the foil picture, so that it travels all over it, from one end to the other. An electrical circuit is arranged so that when the stylus touches a piece of the bare foil a current is sent out along the line wire. This current is therefore intermittent, being interrupted each time the stylus passes over a part of the half-tone picture covered with the non-conducting ink, the succeeding periods of current and no current varying with the breadth of the conducting and the non-conducting lines. This intermittent current goes to a similar arrangement of stylus and cylinder at the receiving end, this cylinder having round it a sheet of paper coated with a chemical preparation. The coating is white all over to begin with, but it turns black wherever the current passes through it. The final result is that