156 



POPULAR SCIENCE MONTHLY. 



At the outset, there was much uncertainty as to the effect of the 

 curvature of the earth on the propagation of a Hertzian wave over a 

 distance of many hundreds of miles. In the case of the Atlantic trans- 

 mission between the station at Poldhu in Cornwall and that at Cape 

 Cod in Massachusetts, U. S. A., we have two stations separated by about 



Fi(i. 26. Wooden Towers Supporting the Marconi Aerial at Poi.dhu Powek Station, 



Cornwall, England. 



45 degrees of longitvide on a great circle, or one eighth part of the cir- 

 cumference of the world. In this case, the versine of the arc or height 

 of the sea at the halfway point above the straight line or chord join- 

 ing the two places is 300 miles. 



This question has recently attracted the attention of several eminent 

 mathematical physicists. The extent to which a free wave propagated 

 in a medium bends round any object or is diffracted depends on the 

 relation between the length of the wave and the size of the object. 

 Thus, for instance, an object the size of an orange held just in front of 

 the mouth does not perceptibly interfere with the propagation of the 

 waves produced by the speaking or singing voice, because these are from 

 two to six feet in length ; but if arrangements are made by means of a 

 Gallon whistle to produce air waves half an inch in length, then an 

 obstacle the size of an orange causes a very distinct acoustic shadow. 

 The same thing is true of waves in the ether. The amount of l)ending 

 of light waves round material objects is exceedingly small, because the 

 average length of light waves is about one fifty-thousandth part of an 



