As we note the progress that the inscriptions show us to have been made beyond the stage reached by prehistoric man, it becomes clear to us that much--if not most--of that progress could not have been made without the aid of writing. One cannot conceive of the invention and development of Astronomy, for instance, without some means of recording observations that had been made. In developing the art of writing itself, much progress was effected in both countries, and many improvements were made in the art itself that must have been due to that lower order of invention which consists in improving on things already existing. In addition, invention was employed in devising and arranging means for preserving the writings in an enduring form. In Babylonia, this was done by making the writing on soft tablets of clay about an inch in thickness, that were afterwards baked to hardness. In the case of records of unusual importance, the precaution was sometimes taken of covering the baked inscription with a thin layer of clay, making a duplicate inscription on this layer, and then baking it also. If afterwards, from any cause, the outside inscription was defaced, it could be removed and the inside inscription exposed to view. In Egypt, the writing was done on sheets of papyrus, made from a reed that grew in the marshes. To devise and make both the baked clay tablets and the papyrus, it is clear that invention had to be employed; for nothing exactly like them existed in nature. Thus the invention of the art of writing was supplemented by the invention of the art of preserving the records that writing made. The act of writing would have been useful, even if no means had been invented for preserving the things written; even if the things written had perished in a day. But the importance of the invention of writing was increased ten thousand fold by the invention of the means for preserving the things written; because without that means it would have been impossible by any process of continual copying of tablets to keep at hand for reference that library of records of the past on which all progress has been based, and from which every act of progress has started, since some inventor of Babylonia invented baked clay tablets and some inventor of Egypt invented papyrus. It may be objected that there is no reason for assuming that any one man invented either; that each invention may have been the joint work of two men, or of several men. This of course, is true; but it does not minimize the importance of either invention, or the credit due to the inventors. It simply divides the credit of each invention among several men, instead of giving it all to one. It is a notable fact, however, that, although some inventions have been made by the joint work of two men, and although some books have been written, and some music has been composed by two men working in cooperation, yet such instances have been rare.