658 HENKY A. KOWLAND 



blackened body absorbs the rays of shorter wave-length from the lamp; 

 and, in both cases, heat is the result. 1 



But in this experiment, as in the first one, the wave-like nature of the 

 disturbance has not been proved experimentally. We have caused elec- 

 tric sparks, and have heated the copper plate across an interval of space, 

 but have not in either of these cases proved experimentally the progres- 

 sive nature of the disturbance; for a ready means of experimenting on 

 the waves, obtaining their wave-length and showing their interferences, 

 has hitherto been wanting. This deficiency has been recently overcome 

 by Professor Hertz, of Carlsruhe, who has made a study of the action of 

 the coil, and has shown us how to use it for experiments on the ethereal 

 waves, whose existence had before been made certain by the mathemat- 

 ics of Maxwell. 



I scarcely know how to present this subject to a non-technical audience 

 and make it clear how a coil of wire with a break in it can be used to 

 measure the velocity and wave-lengths of ethereal waves. However, I 

 can but try. If the waves moved very slowly, we could readily measure 

 the time the first coil took to affect the second, and show that this time 

 was longer as the distance was greater. But it is absolutely inapprecia- 

 ble by any of our instruments, and another method must be found. To 

 obtain the wave-length Professor Hertz used several methods, but that 

 by the formation of stationary weaves is the most easily grasped. Mr. 

 Ames holds in his hand one end of a spiral spring, which makes a very 

 heavy and flexible rope. As he sends a wave down it, you see that it is 

 reflected at the further end, and returns again to his hand. If, how- 

 ever, he sends a succession of waves down the rope, the reflected waves 

 interfere with the direct ones, and divide the rope into a succession of 

 nodes and loops, which you now observe. So a series of sound waves, 

 striking on a wall, form a system of stationary waves in front of the wall. 

 With this in view, Professor Hertz established his apparatus in front of 

 a reflecting wall, and observed the nodes and loops by the sparks pro- 

 duced in a ring of wire. It is impossible for me to repeat this experi- 

 ment before you, as it is a very delicate one, and the sparks produced are 

 almost microscopic. Indeed, I should have to erect an entirely differ- 

 ent apparatus, as the waves from the one before me are nearly % mile 

 long, the time of vibration of the system being very great, that is 

 TWinnnnr ^ a second. To produce shorter waves we must use appa- 



1 The thermopile was connected with a delicate mirror galvanometer, the de- 

 flections of which were shown on a screen. 



