Cambridge, Dec. 16th, 1850.

Dear Col. Sabine,

Perhaps you may recollect speaking to me once at Prof. Miller's about pendulum experiments. In your paper “On the Reduction to a Vacuum of the Vibrations of an Invariable Pendulum,” Phil. Trans. 1829, in speaking of the results to which you had arrived by a comparison of the vibrations in an exhausted receiver, in air at the atmospheric pressure, and in hydrogen, you add (p. 232) “Should the existence of such a distinct property of resistance, varying in the different elastic fluids, be confirmed by experiments now in progress with other gases, &c.” I have not met with any further notice of these experiments, but you told me on the occasion I have mentioned that the experiments showed that the retardation could not at all be inferred from the density, in passing from one elastic fluid to another.

Now I wish to know (supposing, as I believe is the case, that the experiments have not been published) whether you would have any objection to my stating in a paper read before the Cambridge Philosophical Society that I had been informed by you that the experiments here alluded to fully established the existence of a specific action in elastic fluids quite distinct from mere variations of density.

The paper I allude to was read at the last meeting of the Camb. Phil. Soc. It contains the calculation of the resistance to a pendulum in the two cases of a sphere and of a long cylindrical rod, when the internal friction, as it may be called, of the fluid is taken into account. The agreement of theory with Baily's experiments is very striking.