There are various ways of "tuning" receivers and transmitters, but the principle underlying them all is analogous to that of mechanical vibration. If a weight is suspended from the end of a spiral spring, and given an upward blow, it bobs up and down a certain number of times per minute, every movement from start to finish having exactly the same duration as the rest. The resistance of the air and the internal friction of the spring gradually lessen the amplitude of the movements, and the weight finally comes to rest. Suppose that the weight scales 30 lbs., and that it naturally bobs twenty times a minute. If you now take a feather and give it a push every three seconds you can coax it into vigorous motion, assuming that every push catches it exactly on the rebound. The same effect would be produced more slowly if 6 or 9 second intervals were substituted. But if you strike it at 4, 5, or 7 second intervals it will gradually cease to oscillate, as the effect of one blow neutralizes that of another. The same phenomenon is witnessed when two tuning-forks of equal pitch are mounted near one another, and one is struck. The other soon picks up the note. But a fork of unequal pitch would remain dumb. Now, every electrical circuit has a "natural period of oscillation" in which its electric charge vibrates. It is found possible to "tune," or "syntonize," the aerial rod or wire of a receiving station with a transmitter. A vertical wire about 200 feet in length, says Professor J.A. Fleming,[15] has a natural time period of electrical oscillation of about one-millionth of a second. Therefore if waves strike this wire a million times a second they will reinforce one another and influence the coherer; whereas a less or greater frequency will leave it practically unaffected. By adjusting the receiving circuit to the transmitter, or _vice versa_, selective wireless telegraphy becomes possible. ADVANCE OF WIRELESS TELEGRAPHY. The history of wireless telegraphy may be summed up as follows:-- 1842.--Professor Morse sent aerial messages across the Susquehanna River. A line containing a battery and transmitter was carried on posts along one bank and "earthed" in the river at each end. On the other bank was a second wire attached to a receiver and similarly earthed. Whenever contact was made and broken on the battery side, the receiver on the other was affected. Distance about 1 mile. 1859.--James Bowman Lindsay transmitted messages across the Tay at Glencarse in a somewhat similar way. Distance about 1/2 mile. 1885.--Sir William Preece signalled from Lavernock Point, near Cardiff, to Steep Holm, an island in the Bristol Channel. Distance about 5-1/2 miles. In all these electrical _induction_ of current was employed. 1886.--Hertzian waves discovered. 1895.--Professor A. Popoff sent Hertzian wave messages over a distance of 3 miles. 1897.--Marconi signalled from the Needles Hotel, Isle of Wight, to Swanage; 17-1/2 miles.