AT THE VERY beginning of this book we observed that war is a most potent stimulus to invention among primitive men. Despite all our advances in civilization we still have our wars, each more dreadful than the preceding one; for each important conflict brings forth new engines of destruction or stimulates the invention of new death-dealing machines which are developed during intervals of peace. So terrible has modern war become that with each great conflict it has seemed as if its very dreadfulness would stay the hand of the invader and make him hesitate to expose his men to the horrible monsters which science and invention have created. To-day, after a titanic struggle which cost millions of human lives, which destroyed billions of dollars’ worth of property, and which made itself felt to the remotest corners of the earth, serious efforts are being made to banish war, but at the same time inventors here and abroad are busy inventing new and more powerful engines of death, and the prospects are that the next big war, should there ever be one, will be even more horrible and destructive than the one we have just passed through. Fortunately engines of death are not the only inventions produced by war. Many machines, apparatus, and processes are originated or developed which have a distinct value in time of peace; hence war is a great constructive as well as a destructive agency. In this respect the great World War was no exception and we are already reaping benefits which in a small way compensate for the havoc that was wrought. It advanced scientific research to a point it would not have reached in fifty years of peace. The stimulus was felt in every field of science from chemistry to acoustics, from geology to meteorology. Wonderful progress was made in radiotelephony, in submarine navigation, in aviation, and there were scores of inventions which will add to our comfort and aid us in the mastery of Nature. Many of these inventions have already been described; others do not properly belong in this volume as they have to do chiefly with electricity or chemistry. In this chapter we shall take up mainly the machines built for the purpose of destroying men and their works. PRIMITIVE “ENGINES” OF WAR