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P.S. J)t!C. 13.- — In a small model the opposed pieces were two pennies Tound with carborundum to a fit. One of them — the stationary one. — was ftenvards grooved by the file and etched with dilute nitric acid according to ^ig. 3, sealing-wax, applied to the hot metal, being used as a " resist/' They /ere mounted in a small cell of tin plate, the upper one carrying an inertia •ar. With oil as a lubricant, the contrast between the two directions of o tat ion was very marked.
Opportunity has not yot boon found for trying polished glass plates, such ,s are used in optical observations on "interference." hi this case tin; etching k'ould be by hydrofluoric acid*, and air should sufliee as a lubricant.
Comjmro Nature, Vol. i-xiv. p. !5H5 (1.1)01) ;
ijii- Pitpers, Vol. iv. p. C-lfi.    It is unlikely that any great