Johns Hopkins Univ., Baltimore Md.
Dec. 17, 1880

Dear sir,

My paper on the comparison of Absolute Measures of Gravity has not been presented to any public, and I forwarded it to you for such publication as might be judged best. Perhaps it might be inserted in the next report. It relates to work already published in the '76 report.

The pendulum operations instituted for the purpose of ascertaining the vertical attraction of the Allegheny mountains which were commenced in the year 1878 by the occupation of the Allegheny Observatory by the party of Assistant Peirce, have been completed by the occupation of a station at Ebensburg, Cambria County, Pennsylvania, situated upon the top of the ridge of Alleghenies and also of a station at York, Pennsylvania, where similar observations were obtained. A careful determination was made of the difference of longitude between Ebensburg and the Allegheny Observatory, in which Assistant Peirce was aided by Mr. , the assistant at the Allegheny observatory under Professor

S. P. Langley, who kindly placed the resources of the observatory at the disposal of the Survey. Time was also telegraphed from Allegheny to the Ebensburg station daily for the purposes of the pendulum experiments; thus supplementing the time observations made upon the spot. The latitude of the Ebensburg station was determined by a sextant. Assistant Peirce was ably aided at Ebensburg by Mr. Henry Farquhar and Professor Marcus Baker, both of the Survey. The pendulum observations at York were made by Mr. Henry Farquhar under the supervision of Assistant Peirce. At both Ebensburg and York extensive series of observations were made for the purpose of studying the statical and dynamical flexure of variously

The measurement of the acceleration of gravity made by Assistant Peirce in Paris, having upon the reduction of the observations proved to be in violent disaccord with the celebrated measures of the same constant by Borda and by Biot as well as with the results of former transports of invariable pendulums, all of which results were in excellent accord with one another, by the authority of the Hon. Secretary of the Treasury I instructed Assistant Peirce to proceed to Paris for the purpose of investigating the cause of the discrepancy. By theoretical and experimental studies of the supports of Borda and of Biot, Assistant Peirce succeeded in demonstrating from principles of physics

The operations for the comparison of the metre with a wave-length light have come to a provisional completion, although certain portions of the work require to be strengthened and in particular the comparison of the decimeters with the metre remains as yet relatively rough. In connection with this subject Mr. Peirce has with my approval published in the American Journal of Mathematics a memoir, theoretical and observational upon certain apparitions or ghosts which appear in certain diffraction spectrum in consequence of the excentricity of the screw used in valuing the diffraction plates. This paper formed the subject of another by Professor H. C. Rowland of Baltimore, presented to the American Association for the Advancement of Science at their last meeting in Boston.