D O C U M E N T 4 9 5 M A R C H 1 9 2 7 4 8 9 494. Greetings for the Commemoration of Newton’s Bicentenary[1] [Berlin, between 2 March and 18 March 1927][2] [See Doc. 504 in documentary edition for English version.] 495. To Unknown [Berlin,] 6 March 1927 Dear Sir, Your example is very amusing and does you due honor. It is not a matter of “rel- ativ[ity] of energy,” though, but of a thermodynamic machine that transforms heat into work. In essence it is connected with the fact that the specific heat of a magnetizable substance depends on the magnetic field in which it is situated. A thermodynamic analogy to this is that the specific heat (heat capacity) of a gas de- pends on the pressure exerted upon it. (The gas’s volume is thermodynamically analogous to magnetization.) Respectfully yours, A. Einstein P.S. The following is an amusing analogue . [1] cooling [2] heating Wheel with thermally expandable tires and rigid spokes. Right heats up, left cools down. Rotation sustained by gravity. [1] [2]