Mr. Lubbock on the Tides in the Port of London. 125 



I believe many persons suppose that no theory of the tides 

 can be expected to apply to places separated from the main 

 ocean by long and narrowing channels, as is the case in the 

 port of London. This notion will, I think, be removed, if it 

 be considered that the condition which justifies the application 

 of the theory to a river, is not that the tides should there be 

 the same in time or in height as they are in the ocean, but 

 that they should be as regidar in the former as in the latter 

 case ; and that we may here reckon upon this degree of regu- 

 larity at least. Such a situation is in fact, what Laplace con- 

 siders in the case of Brest as a remarkably happy circum- 

 stance for the application of theory. " La situation de ce 

 port (Brest) est tres favorable a ce genre d'observations : il 

 communique avec la mer par un canal fort vaste, au fond du- 

 (juel ce port a ete construit. Les irregularites du mouvement 

 de la mer parvieiment ainsi dans ce port tres afFaiblies : a peu 

 pres comme les oscillations que le mouvement irregulier d'un 

 vaisseau produit dans le barometre sont attenues par un etran- 

 glement fait au tube de cet instrument." — Memoires de Vhi- 

 stitut, 1818: p. 1. 



As long as the bed of the river continues the same, the tide 

 will be regularly transmitted to any point of it, modified only 

 by an alteration of the constant quantities in the formula which 

 the theory gives. This will be the case, even if we suppose the 

 tide which we have here, to be compounded of two tides which 

 arrive by different courses and after different intervals, as 

 might very easily be shown to follow from the nature of the 

 expression by which the height of the water at a given time is 

 represented. The theory of the tides of Laplace in the second 

 volume of the Mecaniq^ie Celeste, is one of the most splendid 

 instances of his unrivalled skill in the application of mathe- 

 matics to questions in physical science; and there can be no 

 doubt that his theory will give the circumstances of the tides at 

 the port of London. Laplace no where hints at any restriction 

 which would exclude such a case, and accordingly it is included 

 with others in the Annuaire du Bureau desLongittides. It is how- 

 ever somewhat remarkable that the table which is given in that 

 work has not undergone any alteration since the time of Ber- 

 nouilli, who first gave it, or has ever, as far as I am aware, 

 been compared with observation. 



This being so, it must, I think, be considered a work of 

 some interest to institute upon a sufficient number of observa- 

 tions a comparison of the theory with the facts: and this is 

 what in the course of last year I executed « itii regard to the 



times 



