456 Intelligence and Miscellaneous Articles, 



also to be discovered by observation. At the London Docks it is 

 small ; but at Liverpool it makes nearly one foot of difference be- 

 tween the superior and inferior tides. This inequality may be seen 

 by inspection of the tables, but it is exhibited more clearly in the 

 zigzag form of the curve traced out by laying down the successive 

 heights of high water as ordinates, and taking the corresponding 

 times as abscissae. A very remarkable peculiarity in the diurnal 

 inequality is, that while the semimenstrual and other inequalities 

 correspond very accurately to the fourth transit of the moon pre- 

 ceding the tide, this inequality corresponds as uniformly to the fifth, 

 a fact which can only be accounted for by supposing (what also 

 appears from other considerations,) that all the circumstances of the 

 tides as observed in the northern hemisphere, depend upon condi- 

 tions existing in the southern hemisphere, and that they are propa- 

 gated from thence northwards, simply by the mechanical laws of 

 undulations in fluids. 



The third lecture was on the comparison of tidal observations in 

 different parts of the world made with a view of tracing the progress 

 of the tide. This theory is entirely of recent origin, being solely 

 attributable to Mr. Whewell, who has justly remarked in his " Essay 

 towards a first approximation to a Map of Cotidal lines," contained 

 in the Philosophical Transactions for 1833*. that no attempt had till 

 tl:en been made to answer decisively the inquiry which Bacon sug- 

 gested to the philosophers of his time, " whether the high water 

 extends across the Atlantic, so as to affect contemporaneously the 

 shores of America and Africa, or whether it is high on one side of 

 this ocean when it is low on the other." The lecturer first showed 

 what would be the motion of the tide wave on a sphere covered 

 with water, and then considered the manner in which it would be 

 affected by continents and islands, inland seas and bays. He also 

 showed how the tide would be obliterated by the propagation of a 

 series of undulations in opposite directions, differing in their epoch 

 by six hours ; and the modifying effects they would mutually pro- 

 duce when they differed by other intervals. The tides of different 

 ports are compared by a comparison of their establishments. The 

 " vulgar establishment" is the time of high water immediately fol- 

 lowing the new or full moon. This involving the semimenstrual 

 inequality as well as the longitudinal difference of the port, a more 

 correct form is obtained by Mr. Whewell in his second essay (Phil. 

 Trans. 1836, p. 293,) by taking the mean of the greatest and least 

 lunitidal intervals, correcting for the moon's parallax and declina- 

 tion, and her motion in JR, and referring the whole to Greenwich 

 time. The establishment thus corrected is called the cotidal hour 

 of the port. 



At the instance of Mr. Whewell, a request was made by the 

 British Association for the Advancement of Science, to the British 

 Government to procure a series of simultaneous tide observations to 

 be made on the shores of Europe and America, in order to ascertain 

 accurately the places of contemporaneous high water. In pur- 

 • See p. 354 of the present volume. 



