DISCOVERY 



115 



In this connection, for instance, it is possible that the 

 result of recent observations of the sun's ecUpse may 

 require some modifications to be made in certain 

 assumptions regarding the transference of energy 

 by ether waves. 



The energy which produces sound is also transmitted 

 by wave motions of a medium, which in this case, 

 however, is gaseous (as air), or solid or liquid. 



The difference in the media in the cases of sound 

 and light can be readily demonstrated by connecting 

 up an electric bell and its battery under the receiver 

 of an air-pump so that the space surrounding the bell, 

 enclosed in the glass receiver, can be exhausted as far as 

 jKjssible of air. It will be found that, as the air becomes 

 more and more rarefied, the sound of the bell decreases, 

 eventually ceasing altogether when the receiver is 

 exhausted of air. The hammer, however, can still be 

 seen striking the bell, which shows that light is tra- 

 versing the e.xhausted space, whereas sound is not. 



The method by which energy can be transferred by 

 wave motions in a medium can be seen by dropping 

 a stone into a pool of water. The energy imparted to 

 the water by the stone is radiated in the form of a 

 circle of waves, the circle increasing in size, and the 

 heights of the waves becoming smaller, until they reach 

 the edges of the pool, or die away altogether. Now, if 

 some corks are floating in the pool, the energy- imparted 

 to the water by the stone, and propagated bv the waves, 

 will cause the corks to bob up and down, thus showing 

 that the little particles of water, which are in this case 

 the medium, merely oscillate up and down, and by 

 doing so, convey energy in the form of waves, the corks 

 '•arer the original disturbance receiving more energy 

 :an those farther away. 



The stone, by disturbing the water at one place, has 

 thus transferred energy to other places by waves in 

 the medium, the water, without any transference of 

 matter taking place. 



Similarly, if one end of a piece of rope is fixed, and 

 the other end is rapidly moved up and down by hand, 

 energy is transferred by wave motions in the rope from 

 the hand to the place where the rope is attached 

 without any matter being transferred from the one 

 end of the rope to the other. 



In both these cases the wave motion by which energy 

 is transferred is produced by oscillation of the little 

 particles which make up the medium — that is, the water 

 or the rope, up and down at right angles to the hne of 

 propagation of the energy. A wave length is the dis- 

 tance separating the crests of adjacent waves, and it 

 will be apparent, by observing the motions of the rope, 

 that the more rapid the oscillation the shorter are the 

 wave lengths, and vice versa. 



It is in just such a manner that light and heat are 

 transmitted from, say, the sun to the earth, except 



that the oscillations of the medium, in this case the 

 ether, take place not only in one direction, but in all 

 directions at right angles to the hne of propagation. 



If the oscillations of the ether are at certain rates, 

 they affect the eye and produce the sensation of light ; 

 if at certain other rates, they are received not as light, 

 but as heat. 



If the ether is set in oscillation at very much slower 

 rates than can be detected as light or heat, the ether 

 waves produced are those which are used for com- 

 munication by Wireless Telegraphy. These wireless 

 waves are often called Hertzian Waves, after Hertz, 

 a German physicist, who in 1878 pubUshed the results 

 of his experiments on the production and properties 

 of these waves, results which confirmed theoretical 

 deductions pubhshed in this country by Clark Ma.xwell 

 in 1864. 



The transference of energy by all these ether waves, 

 Hght, heat, and wireless, takes place at the same rate, 

 viz. 186,000 miles a second ; but the rates of oscillation 

 of the ether on which depend the lengths of the waves 

 differ very much in each case. 



The highest known rates of oscillation in the ether 

 produce Rongten or X rays, then come the actinic rays, 

 which are used in photography, and then the \dsible 

 hght rays, at rates of oscillation from about 1,200 

 biUions a second for violet light, down through indigo, 

 blue, green, yellow, and orange, to about 600 billions 

 a second for red. A combination of all these colour 

 rays produces white hght, i.e. sunlight. Below these 

 rates of oscillation are the heat rays, and then the 

 rates of oscillation, about 3 miUions to 15,000 a second, 

 which produce the Hertzian waves now used for wireless 

 signalhng. 



These wireless waves are therefore very similar to 

 heat or light waves, and travel at the same speed, the 

 main difference being that the rates of oscillation of 

 the ether which produce them, though so exceedingly 

 rapid, are far too slow to produce the sensation of 

 heat or hght ; in other words, these wireless waves can 

 be neither felt nor seen. Neither, of course, can they 

 be heard, as they have no connection whatever with 

 audible sounds, which are produced, not by ether 

 oscillations, but by air oscillations, acting on the drum 

 of the ear. 



The speed of travel of ether waves, 186,000 miles a 

 second, is so great as to be, for all practical purposes, 

 instantaneous so far as propagation over the earth is 

 concerned ; though when the stupendous distances dealt 

 with in astronomy are considered, this speed is, of 

 course, far from instantaneous. For example, it takes 

 some eight minutes for light from the sun, and about 

 four years for light from the nearest star, to reach the 

 earth. 



Signalling by an unscreened flashing lamp may be 



