With respect to many forces of nature, the unaided senses of man are helpless. The subtle force of magnetism, for instance, appears incapable of affecting directly any of the senses. A person may hold a powerful lodestone in his hand and feel no influence different from that coming from a piece of sandstone. A person may work near a wire carrying a current of electricity, and, though it is well known that peculiar conditions exist in the universal ether around such a wire, yet, through his five senses, he may never become aware of the existence of this current. A piece of uranium ore, as has been found in recent years, emits various kinds of rays related to the now famous X- or Roentgen rays, yet no indication comes directly through any of the five senses that such is the case. In fact, men of science worked with the ores of uranium for many years before discovering the emission of ether waves. In the light which comes from the sun are numerous forms of energy that do not directly affect the senses, and therefore remained unknown for many centuries. Numerous other illustrations might be quoted to show the existence of natural forces that are beyond the direct recognition of man. In the great ocean of the unknown, lie, undoubtedly, countless forces that shall never be known by a direct action upon the senses of man.[A] [Footnote A: The writer is aware of the beliefs held by many students regarding the so-called touch sense, heat sense, magnetic sense, electrical sense, spiritual sense, etc. So little is known of these subdivisions of the sense of feeling, that they are not considered in this popular writing. There is, moreover, no evidence that the magnetic sense, as an example, if it exists, is a direct effect of magnetic forces; it is as easily believed that the body somehow converts magnetic forces, under certain circumstances, into other forces that may be sensed by man.] [Sidenote: The advance of knowledge requires instruments that convert natural phenomena into intelligible forms. Thus the unknown is revealed.]