. Annual report of the Board of Regents of the Smithsonian Institution . ^ of the balls. If the rotating system C and D had been invisible we should haveexplained the behavior of thesystem by assuming an at-tractive force with corre-sponding potential energy be-tween A and B. This is dueto our considering A and B asa complete system, whereas itis in reality part of a largersystem, and when we consider the complete system we see that itbehaves as if it were acted on by no forces and possessed no energyother than kinetic. It may perhaps be of interest to note that we can in a similar waymake two
. Annual report of the Board of Regents of the Smithsonian Institution . ^ of the balls. If the rotating system C and D had been invisible we should haveexplained the behavior of thesystem by assuming an at-tractive force with corre-sponding potential energy be-tween A and B. This is dueto our considering A and B asa complete system, whereas itis in reality part of a largersystem, and when we consider the complete system we see that itbehaves as if it were acted on by no forces and possessed no energyother than kinetic. It may perhaps be of interest to note that we can in a similar waymake two bodies appear to attract each other with a force varyinginversely as the square of the distance between them. Let A and B(fig. 2) be the bodies, and suppose that parabolic wires without mass Fig. BIG. 244 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1908. are fixed to them, if these are threaded through a ring P with a smallbut finite mass and the system caused to rotate round A and B, theeffort of the ring to get away from the axis of rotation will causeA and B to approach each other, and the law of approach may easilybe shown to be the same as if there was a force between them vary-ing inversely as the square of the distance. The result mentioned on page 236 that the potential energy of asystem charged with electricity is equal to the kinetic energy of themass of ether bound to the system when moving with the velocityof light is another example of potential energy, being in reality thekinetic energy of an associated system, and indeed, as I have endeav-ored to bring before you this evening, the study of the problemsbrought before us by recent investigations leads us to the conclusionthat ordinary material systems must be connected with invisiblesystems which possess mass whenever
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