There is no reason to doubt that Bacon's investigations led him to discoveries of essential value, but which for the most part died with him. His active and piercing intellect, which employed itself on the most difficult subjects, which led him to the formation of a theory of tides, and brought him to see the need and with prophetic anticipation to point out the means of a reformation of the calendar, enabled him to discover many of what were then called the Secrets of Nature. The popular belief that he was the inventor of gunpowder had its origin in two passages in his treatise "On the Secret Works of Art and Nature, and on the Nullity of Magic,"[37] in one of which he describes some of its qualities, while in the other he apparently conceals its composition under an enigma.[38] He had made experiments with Greek fire and the magnet; he had constructed burning-glasses, and lenses of various power; and had practised with multiplying-mirrors, and with mirrors that magnified and diminished. It was no wonder that a man who knew and employed such wonderful things, who was known, too, to have sought for artificial gold, should gain the reputation of a wizard, and that his books should be looked upon with suspicion. As he himself says,--"Many books are esteemed magic, which are not so, but contain the dignity of knowledge." And he adds,--"For, as it is unworthy and unlawful for a wise man to deal with magic, so it is superfluous and unnecessary."[39] There is a passage in this treatise "On the Nullity of Magic" of remarkable character, as exhibiting the achievements, or, if not the actual achievements, the things esteemed possible by the inventors of the thirteenth century. There is in it a seeming mixture of fancy and of fact, of childish credulity with more than mere haphazard prophecy of mechanical and physical results which have been so lately reached in the progress of science as to be among new things even six centuries after Bacon's death. Its positiveness of statement is puzzling, when tested by what is known from other sources of the nature of the discoveries and inventions of that early time; and were there reason to question Bacon's truth, it would seem as if he had mistaken his dreams for facts. As it stands, it is one of the most curious existing illustrations of the state of physical science in the Middle Ages. It runs as follows:--"I will now, in the first place, speak of some of the wonderful works of Art and Nature, that I may afterwards assign the causes and methods of them, in which there is nothing magical, so that it may be seen how inferior and worthless all magic power is, in comparison with these works. And first, according to the fashion and rule of Art alone. Thus, machines can be made for navigation without men to row them; so that ships of the largest size, whether on rivers or the sea, can be carried forward, under the guidance of a single man, at greater speed than if they were full of men [rowers].