III. Note on the tides

Abstract

By the permission of Mr. Dessiou, I am enabled to communicate to the Society some results which he has obtained, from observations made at Plymouth, Portsmouth and Sheerness, under the superintendance of the masters attendant at those dock-yards. The establishments of these ports (that is, the time of high water when the moon passes the meridian at 12 o’clock,) may now be considered as accurately determined. The retard (λ—λ 1 ) at Portsmouth appears to be intermediate between that at Brest and at London, being about 1 h 30 m ; that at Plymouth appears to be greater, and not less than that which obtains at London, for which circumstance it is difficult to account. The retard at Sheerness agrees with that observed at the London Docks. The constant which involves the mass of the moon does not differ much, as inferred from these observations at various places; it is however impossible to obtain more than a rough approximation, by these means, to that important element in astronomy, the mass of the Moon. Even a minute (of time) in the difference of the interval between the moon’s transit and the corresponding time of high water, materially affects the value of the moon’s mass; and insurmountable difficulties appear to be in the way of any nice determination of that quantity by these means, even if there were none of an analytical character. It would be well, by a discussion of the inevitable errors of the various data employed, to ascertain the limits of the errors which may be incurred in determining the mass of the moon by various methods, particularly through the constant of the moon’s parallax. Mr. Dessiou, with undaunted perseverance, has just completed the discussion of about 6000 more observations of the tides at the London Docks, with a view to rest upon a sure basis the corrections for the moon’s parallax and declination; but these cannot be published unless he is fortunate in meeting with more encouragement than he has hitherto experienced. These results do not differ materially from those already published in the Phil. Trans. 1831, Part II. p. 413, the accuracy of which therefore they serve to confirm.