18 THE REV. W. WHEWELL ON THE EMPIRICAL LAWS 



are calculated according to rules obtained by Mr. Holden, some years ago, from the 

 examination of five years of observations made at the Liverpool Docks by Mr. Hut- 

 chinson, at that time harbour-master. The calculations are at present conducted by 

 the Rev. George Holden, of Maghull, a descendant of the person who first invented the 

 rules. Other Liverpool tide tables are also calculated by Mr. Woffinden. Of London 

 tides several, apparently independent, tables are annually published ; and though the 

 differences of these are considerable, I do not know that any one set is considered as 

 possessing a decided superiority in the general result. I am not aware that any tide 

 tables are published for Brest, though so large a collection of observations has been 

 made at that port, and though so much labour has been employed in the discussion 

 of these, for the purpose of comparing certain points of Laplace's theory with them : 

 nor have, I believe, tide tables for any place been calculated according to the method 

 recommended in the M^canique Celeste. 



The method generally practised in England for the construction of tide tables for 

 other places has been, to take the time which is stated in the London or the Liverpool 

 tables, and, if necessary, to add or subtract some constant quantity, according to the 

 place. The Liverpool tide tables are in this manner used, generally without correction, 

 for the whole of the north-western coast of England : and tables are published pro- 

 fessing to give the hours at most of the principal ports of England, in parallel columns ; 

 the hours at different places having constant differences. Thus the hour of high water 

 at Plymouth is stated as always 1^ 55°^ later than the hour in the same half-day at 

 London. This assumption of a constant difference in the hours of high water at dif- 

 ferent places is, however, inexact ; as we should expect it to be from considering the 

 mode in which the tide is transmitted from one place to another, and as it appears 

 to be from observation. 



It appears, therefore, that the most promising mode of advancing our knowledge of 

 the tides, is to examine the laws which can be collected from observation, taking so 

 great a number of observations, that the effects of all accidental causes may disappear 

 in the average results. The collection of observations discussed by Mr. Dessiou, under 

 the direction of Mr. Lubbock, affords us an admirable opportunity for this examination ; 

 the collection including 13073 observations, and a period of nineteen years, from Janu- 

 ary 1st, 1808, to December 31st, 1826. Our object in this examination being to ascer- 

 tain the manner in which the positions and distances of the heavenly bodies affect the 

 time and height of high water, the mode of proceeding must be to examine how these 

 two quantities depend upon the right ascension, declination and parallax of the sun 

 and moon, and upon other astronomical elements, if such are found to be needed. 

 The mean time of high water will be found to be affected by inequalities, depending 

 on the elements just mentioned; and the law and amount of these inequalities may 

 be collected from observations, without any reference to theory, (provided the obser- 

 vations are sufficiently numerous and their circumstances sufficiently varied,) in the 

 same manner in which the greater inequalities of the moon, the variation, evection and 

 annual equation, were detected by observation, long before the motions of the heavenly 



