A continual effort is necessary to control magnetism, and it can be controlled by a certain amount of reserve. But there are two things to consider. By too much reserve one will also close the doors to the development of magnetism, just as too much work will exhaust a man while working moderately will develop his muscular system. So it is with magnetism: when we use it continually it is soon exhausted, but when it is used no more than is needed in order to cultivate it and control it then it remains in proper proportion.