lowly 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.

1901.--Messages sent at sea for 380 miles.

1901, Dec. 17.--Messages transmitted from Poldhu, Cornwall, to Hospital
Point, Newfoundland; 2,099 miles.

Mr. Marconi has so perfected tuning devices that his transatlantic
messages do not affect receivers placed on board ships crossing the
ocean, unles