ANALYSIS OF THE IONOSPHERE 457 



is chiefly famed for bridging great distances over the earth, but its 

 foundations are best studied with sender and receiver side by side. 



Electromagnetic signals thus find a mirror or a ceiling overhead; 

 and the theory interprets this mirror as consisting of the ions, and 

 especially the free electrons, diffused in the upper air. This perhaps 

 seems singular, in view of the tenuity of the air and the lightness of 

 the individual electrons. It might have seemed better, at least in the 

 days of the Greeks, to propose that the dome of the sky is a hard 

 metallic mirror— of well-polished silver, for instance.. Well, in effect 

 that is what is proposed. A mirror of silver reflects not by virtue of 

 its hardness, but because of electrons diffused like a gas through the 

 pores of the metal. A metal is a container for an electron-gas, and in 

 the upper air there is an electron-gas without a container; and both 

 of them reflect. 



The theory is strictly classical, in the sense of the word prevailing 

 in physics. No relativity, no quantum theory, no suggested revision 

 of the concepts of space and time, afflict the student thereof. It is 

 the working-out of the basic principle of Maxwell and Lorentz, that 

 the passage of electromagnetic waves through a medium is controlled 

 by the electric current which the waves themselves evoke in the 

 medium. Under the influence of the electric field in the waves, the 

 ions swing in sympathetic vibration, and form a part of that current. 

 They thus react upon the waves, alter the speed thereof, and bring 

 about the reflection. The motion of the ions is simple-harmonic, so 

 that the mathematics of the theory is simple and familiar — so long, 

 at least, as no account is taken of any forces acting on the ions other 

 than that due to the field of the waves themselves. Here is the 

 explanation of the echoing of radio signals, and hence follows the 

 procedure for translating the data of echoes into statements about 

 the distribution of the ions in the atmosphere. It is not difficult to 

 describe or explain, and will be carried through in this article. 



From this point the theory ramifies in two directions. Two things 

 modify the sympathetic vibrations of the ions: the collisions between 

 the ions and the neutral molecules of the air, and the earth's magnetic 

 field. By their influence on the vibrations, they modify the speed 

 of the waves, and therefore the conditions of the echoing. The 

 theory extended in either direction continues to be easy in one sense, 

 for neither the physical concepts nor the mathematical operations are 

 unfamiliar; but becomes very hard in another, for the algebraic 

 expressions are often of fearful complexity, impossible to remember 

 and hard even to keep straight when written out. When it is extended 

 in both ways at once the expressions become so intricate, that nearly 



