Researches on the tides. Seventh series. On the diurnal inequality of the height of the tide, especially at Plymouth and at Sincapore : and on the mean level of the sea

Abstract

The diurnal inequality which the author investigates in the present paper, is that by which the height of the morning tide differs from that of the evening of the same day; a difference which is often very considerable, and of great importance in practical navigation, naval officers having frequently found that the preservation or destruction of a ship depended on a correct knowledge of the amount of this variation. In the first section of the paper he treats of the diurnal inequality in the height of the tides at Plymouth, at which port good tide observations are regularly made at the Dock Yard ; and these observations clearly indicate the existence of this inequality. As all the other inequalities of the tides have been found to follow the laws of the equilibrium theory, the author has endeavoured to trace the laws of the diurnal inequality by assuming a similar kind of correspondence with the same theory; and the results have confirmed, in the most striking manner, the correctness of that assumption. By taking the moon’s declination four days anterior to the day of observation, the results of computation accorded, with great accuracy, with the observed heights of the tides: that is, the period employed was the fifth lunar transit preceding each tide. In the second section, the observations made on the tides at Sincapore from August 1834 to August 1835, are discussed. A diurnal inequality was found to exist at that place, nearly agreeing in law and in amount with that at Plymouth ; the only difference being that, instead of four days, it was found necessary to take the lunar declination a day and a half preceding the tide ; or, more exactly, at the interpolated,or north lunar transit, which intervened between the second and third south transit preceding the tide. The diurnal inequality at Sincapore is of enormous magnitude, amounting in many cases to six feet of difference between the morning and evening tides; the whole rise of the mean tide being only seven feet at spring tides, and the difference between mean spring and neap tides not exceeding two feet.