CHAPTER XIV.
THE ACTIVITY OF CHILDREN.
In order rightly to understand the true nature of that extraordinary activity which is so noticeable in all children that are in a state of health, so as to be able to deal with it on the right principles and in a proper manner, it is necessary to turn our attention somewhat carefully to certain scientific truths in respect to the nature and action of force in general which are now abundantly established, and which throw great light on the true character of that peculiar form of it which is so characteristic of childhood, and is, indeed, so abundantly developed by the vital functions of almost all young animals.

One of the fundamental principles of this system of scientific truth is that which is called the persistence of force.
_The Persistence of Force_.
By the persistence of force is meant the principle--one now established with so much certainty as to command the assent of every thinking man who examines the subject--that in the ordinary course of nature no force is either ever originated or ever destroyed, but only changed in form.
In other words, that all existing forces are but the continuation or prolongation of other forces preceding them, either of the same or other forms, but precisely equivalent in amount; and that no force can terminate its action in any other way than by being transmuted into some other force, either of the same or of some other form; but still, again, precisely equivalent in amount.
It was formerly believed that a force might under certain circumstances be _originated_--created, as it were--and hence the attempts to contrive machines for perpetual motion--that is, machines for the _production_ of force.

This idea is now wholly renounced by all well-informed men as utterly impossible in the nature of things.

All that human mechanism can do is to provide modes for using advantageously a force previously existing, without the possibility of either increasing or diminishing it.

No existing force can be destroyed.