F. C. Bell in his contribution raises most interesting philosophical questions. In commenting on them, I will refer to a recent translation (Navigation U.S.A., 14, 26) of Schuler's paper and use the same notation.

The impossibility of distinguishing by any physical measurement between a gravitational force and the inertial reaction force of an accelerating frame of reference is fundamental. In Fig. 1 our observer in an enclosed laboratory is observing the compression of a linear spring supporting a ‘proof mass’ m (he knows the natural uncompressed length of the spring).