Page:Popular Science Monthly Volume 64.djvu/160

156 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 transmission between the station at Poldhu in Cornwall and that at Cape Cod in Massachusetts, U. S. A., we have two stations separated by about

45 degrees of longitude on a great circle, or one eighth part of the circumference 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 joining 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 bending of light waves round material objects is exceedingly small, because the average length of light waves is about one fifty-thousandth part of an