Let us realize the extreme improbability that the phenomenon of the expulsive force of gunpowder was then noted for the first time. It seems probable that accidental ignition of the mixture had often occurred before, and missiles hurled in all directions in consequence. But, as happens in the vast majority of all incidents, no one imagined any possible utilization of the facts disclosed by the incident; and if the man who invented the gun, after witnessing the expulsion of the pestle from the mortar, had not been endowed with both imagination and constructiveness, he would have treated it as most of us treat an incident--merely as an incident. But the imagination of this man must at once have conceived a picture of what we now call a mortar, which should be designed and constructed so that projectiles could be expelled from it at will, in whatever direction the mortar were pointing; and then his constructive faculty must have taken up the task that imagination had suggested, and developed the conception into a concrete thing. Into the long, elaborate and exciting history of the development of the gun, that has been carried on with enormous energy ever since, it is not necessary at this point to enter. Since the sixteenth century, its history is accurately known, and many large books are filled with descriptions and diagrams and mathematical tables and formulæ that recount its progress in detail; while the histories of all the nations blaze with stories of the battles in which guns have been employed. Of all the inventions ever made, it is doubtful if the development and improvement of any other has enlisted the services of a greater number of men and of more important men, than the gun. It is more than doubtful if a greater amount of money has been expended on any other invention, if a greater number of experiments have been made, or if more mental and physical energy has been expended. Certain it is that no other invention has had so direct and powerful an effect on human beings; for the number of men it has killed and wounded must be expressed in terms of millions.