Now if two sympathetic nerve systems operated upon by psychical stimuli be directed to one and the same point, it is by no means difficult to understand how the brains belonging to those systems may be brought into telegraphic communication by means of the nerve fibres, the product of the two minds evolved, and the resultant idea, by means of a simple mechanical contrivance operated upon by the motor function already explained, be transmitted to paper by the process of writing so familiar to both. The action of the psychical stimuli on the nerve fibre, and its transmission thence to the muscles resulting in the movement of the board, is so subtle that we ourselves are not aware of its operation except through the results produced. It has just been said that two minds may be brought into telegraphic communication by means of nerve fibres. Let us see how far the expression is justified by facts. There are few of us who have not experienced the truth of Solomon's saying that "if two persons lie together, they have _heat_; but how can one be warm alone?" Even the close proximity of two persons affects their respective temperatures, and heat and motion we know to be correlative. It has been shown by the physicist that mechanical force producing motion is correlative with and convertible into heat, heat into chemical force, chemical force into electrical force, and electrical force into magnetic force. Moreover, that each of these is correlative and convertible into the other, all being thus interchangeable.