HERTZIAN WAVE WIRELESS TELEGRAPHY. 99 



say of a hundred a second, they give rise to aerial oscillations in the 

 tube, which impress the ear as a deep, musical note or roar; and this 

 continuous sound can be cut up by closing the valve intermittently into 

 long and short periods, and so caused to signal a letter according to 

 the Morse code, denoting the name of the lighthouse. In this case 

 the object is to produce : first, aerial vibrations in the tube, giving rise 

 to a train of powerful air waves; secondly, to intermit this wave-train 

 so as to produce an intelligible signal; and thirdly, to transmit this 

 wave as far as possible through space. 



The production of a sound or air wave can only be achieved by 

 administering a very sudden blow to the general mass of the air in the 

 tube. This impulse must be sufficient to call into operation the inertia 

 and elastic qualities of the air. It is found, moreover, that the ampli- 

 tude of the resulting wave, or the loudness of the sound, is increased 

 by suitably proportioning the length of the siren pipe and the fre- 

 quency of the air puffs ; whilst the distance at which it is heard depends 

 also in some degree upon the form of the mouthpiece. 



Inside the siren tube, when it is in operation, the air molecules are 

 in rapid vibratory motion in the direction of the length of the tube. 

 If we could at any one instant examine the distribution and changes 

 of air pressure in the tube, we should find that at some places there 

 are large, and at others small, variations in air pressure. These latter 

 places are called the nodes of pressure. At the pressure nodes, how- 

 ever, we should find large variations in the velocity of the air particles, 

 and these points are called the antinodes of velocity. In those places 

 at which the pressure variation is greatest, the velocity changes are 

 least, and vice versa. Outside the tube, as a result of these air motions 

 in it, we have a hemispherical air wave produced, which travels out 

 from the mouthpiece as a center; and if we could examine the distri- 

 bution of air pressure and velocity through all external space, we should 

 find a distribution which is periodic in space as well as time, consti- 

 tuting the familiar phenomenon of an air wave. 



Turning then to consider the production of an electric, instead of 

 an air wave, we notice in the first place that the medium with which 

 we are concerned is the ether filling all space. This ether permits the 

 production of physical changes in it which are analogous to, but not 

 identical in nature with, the pressures and movements which constitute 

 a sound wave. The Hertzian radiator is an appliance for acting on 

 the ether as the siren acts on the air. It produces a wave in it, and it 

 can be shown that all the parts of the above described siren apparatus 

 have their electrical equivalents in the transmitter employed in Hertz- 

 ian wave wireless telegraphy. 



To understand the nature of an electric wave we must consider, in 

 the first place, some properties of the ether. In this medium we can 



