Again, the direction of the line is the result of a movement of your hand and arm. Very likely you moved uncertainly: you were not even sure in what direction it was moving. But, if you were a skilful and practised draughtsman, don’t you suppose you could so regulate the movement of your hand and arm, that the line would take the exact direction you desired? Yes, you would have as much control over the direction and character of the line, as a musician has over the keys of a piano, over which his hands move in various directions, sounding the various notes.