Such illustrations might be multiplied, but would add no strength to the discussion. There is, however, another class of instruments which enable the senses to recognize natural forces that do not act directly upon the consciousness of man. If a musical note is produced on a violin, near a piano, the piano string which is stretched or tuned right, will give out the same note. The sound waves from the violin penetrate the piano, and the string which is tuned to give out the same note takes up the energy of the sound waves, and is set in vibration, with the result that the same note is given out by the piano. This is known as sympathetic vibrations. It is possible, therefore, to make a piano give out any note within its range, without any solid object touching the instrument. In the universal ether, which surrounds and penetrates all things, are numberless waves of all kinds, and of all vibrations. If the proper instrument be used, and tuned aright, it is possible to separate from this tumult of waves any desired kind or degree of wave motion, and to convert it into some known form of energy, say electricity. This principle is used in modern wireless telegraphy. Electric waves are sent out by the operator with a certain rapidity. These waves radiate into space, in all directions, and are lost, apparently, in the confusion of myriads of other waves. Nevertheless, if the waves are not by some chance totally destroyed, it is possible to obtain them again, by the use of a receiving instrument which is tuned exactly the same as that used by the operator, at the station where the waves are sent out. A message sent from London may be received anywhere on earth where the receiving instruments are tuned aright; at the same time, if the peculiar note or vibration of the message is not known, so that the receivers can not be tuned properly, the message, though it be all about it, can never be received. Such aids to our senses do not depend so much upon the nature of the material, as upon the degree to which it is brought into sympathy with the force to be recognized. [Sidenote: With proper aids man's senses may discover the whole of nature.] Now, though our senses are imperfect, and recognize only a small part of the phenomena of nature, yet it is very probable that, with such helps as have been described, nothing in nature need remain forever unknown. The means by which the forces of nature, that cannot be sensed directly, are brought to man's recognition may well be named, collectively, man's sixth sense. The progress of science depends upon the discovery of aids to man's senses; a new and vast field is invariably opened whenever a new aid is discovered. [Sidenote: Joseph Smith recognized the existence of media which render the unknown, known.]