A CENTURY'S PROGRESS IN PHYSICS 369 



able fact that the equations describing the motion of 

 Kelvin's quasi-labile ether are of exactly the same form 

 as the electromagnetic equations. Electric displacement 

 is represented by an actual displacement of the ether, 

 magnetic intensity by a rotation. Hence everything 

 which can be explained by the electrodynamic equations 

 finds an analogue in terms of Kelvin's ether. Still 

 another type of dynamic ether which fits the known facts 

 was proposed by McCullagh and perfected by Larmor. 

 In this ether a rotational elasticity is premised, such as 

 would exist if each particle of the medium consisted of 

 three rigidly connected gyrostats with mutually perpen- 

 dicular axes. In this ether electrical displacements cor- 

 respond to rotations, and magnetic strains to etherial dis- 

 placement. 



A New Point of View. While the dynamical school 

 was still dominant in England, another point of view 

 was developing on the continent. Kirchhoff denied 

 that it was the province of science to provide mechanical 

 explanations of the ether and electrodynamic phenomena 

 such as Kelvin conceived to be necessary in order to make 

 these phenomena intelligible. Kirchhoff's contention 

 was that the object of science is purely descriptive, 

 phenomena must be observed, classified, and mutual con- 

 nections described by the fewest number of differential 

 equations possible. Mach expressed the same idea 

 somewhat more concisely when he asserted that the aim 

 of science is "economy of thought." For instance, in 

 the time of Newton, planetary motions could be described 

 quite satisfactorily by means of the three laws of Kepler. 

 The motion of falling bodies on the earth's surface had 

 been described with a fair degree of accuracy by Galileo. 

 The value of Newton's law of gravitation, however, lay in 

 the fact that this great generalization made it possible to 

 describe these and many other types of motion by a 

 single simple formula, instead of leaving each to be gov- 

 erned by a number of separate and apparently unrelated 

 laws. The importance of such a generalization is meas- 

 ured by the economy of thought which it introduces. 



Electron Theory. The electron theory was leading to 

 a reversal of Kelvin's idea that dynamical principles 



