But of course there is a severe limitation to the usefulness of this machine, inasmuch as the drawing has to be made at the time of transmission, and it can only be "put on the wire" by the hand of the artist himself. CHAPTER XIV A WONDERFUL EXAMPLE OF SCIENCE AND SKILL In the preceding chapter reference was made to the fact that for the successful sending of pictures "by wire" one thing was necessary above all others. That one thing consists in making two machines, perhaps hundreds of miles apart, start working together, stop together and, when working, turn at exactly the same speed. Let the reader just picture the problem to himself, and ask himself how such an arrangement can be possible. Let him think of a town two hundred miles away and then meditate on the possibility of making a machine working in his own room and another in that distant town maintain perfect unanimity in their movements. The result of such reflection will probably be the assertion that such a thing is beyond the bounds of possibility. Then he will find the following description of how it is done extremely interesting. In the first place it must be understood that each machine is driven by an electric motor. The motors are designed to run at 3000 revolutions per minute, and they drive the cylinders of the machines through gearing so arranged that the latter turn at 50 revolutions per minute. Now of all machines perhaps the most docile and easily managed is the direct-current electric motor. Each such machine is made with a view to its working at a certain speed, but that can be varied within certain limits, by simply varying the force of the current which drives it. And that force can be very easily varied by the use of an instrument called a "rheostat" or variable resistance. We are all familiar with the way in which the engine-driver regulates the speed of a locomotive, by means of a valve in the steam-pipe. The opening and closing, more or less, of the valve enables the speed to be changed at will and adjusted to a nicety. The rheostat is to the electric current what the valve is to the steam; it can be opened and closed, more or less, as necessary. By it the current driving the motor can be made stronger or weaker, and as that change is made so does the speed of the motor change accordingly. Thus we see that there is at hand the means of setting a motor to work at any desired speed.