Now, it is generally true that whereas matter of any kind offers resistance to light--that is, is not so perfect a conductor as the ether--many substances, especially metals, are more sensitive than ether to heat and electricity. How quickly a spoon inserted into a hot cup of tea becomes uncomfortably hot, though the hand can be held very close to the liquid without feeling more than a gentle warmth. And we all have noticed that the very least air-gap in an electric circuit effectively breaks a current capable of traversing miles of wire. If the current is so intense that it insists on passing the gap, it leaps across with a report, making a spark that is at once intensely bright and hot. Metal wires are to electricity what speaking tubes are to sound; they are as it were electrical tubes through the air and ether. But just as a person listening outside a speaking tube might faintly hear the sounds passing through it, so an instrument gifted with an “electric ear” would detect the currents passing through the wire. Wireless telegraphy is possible because mankind has discovered instruments which act as _electric ears or eyes_, catching and recording vibrations that had hitherto remained undetected. The earliest known form of wireless telegraphy is transmission of messages by light. A man on a hill lights a lamp or a fire. This represents his instrument for agitating the ether into waves, which proceed straight ahead with incredible velocity until they reach the receiver, the eye of a man watching at a point from which the light is visible. Then came electric telegraphy. At first a complete circuit (two wires) was used. But in 1838 it was discovered that if instead of two wires only one was used, the other being replaced by an earth connection, not only was the effect equally powerful, but even double of what it was with the metallic circuit. Thus the first step had been taken towards wireless electrical telegraphy. The second was, of course, to abolish the other wire. This was first effected by Professor Morse, who, in 1842, sent signals across the Susquehanna River without metallic connections of any sort. Along each bank of the river was stretched a wire three times as long as the river was broad. In the one wire a battery and transmitter were inserted, in the other a receiving instrument or galvanometer. Each wire terminated at each end in a large copper plate sunk in the water. Morse’s conclusions were that provided the wires were long enough and the plates large enough messages could be transmitted for an indefinite distance; the current passing from plate to plate, though a large portion of it would be lost in the water.[1]