THE REV. W. WHEWELL'S RESULTS OF TIDE OBSERVATIONS. 85 



intervals of the moon's transit and high water, and drawing a continuous line through 

 the extremities of these ordinates. The curves present, in general, the form of that 

 deduced by Mr. Lubbock from the London Observations, though of course in the 

 rude observations of a single fortnight there are great irregularities. But the means 

 of several places, and even, in most instances, the tides of a single place, present the 

 features of agreement with theory, which Mr. Lubbock has shown to obtain with 

 such remarkable exactness in the London tides ; that is, the ordinate of the curve 

 has in the course of a fortnight a minimum and a maximum magnitude, so that the 

 curve assumes the form jt- Moreover, it is not symmetrical on the two sides of the 

 minimum and maximum, the slope being greater after the minimum than before it. 

 The curve descends from the 7th to about the 13th of June; it then ascends till the 

 18th or 19th, and ascends more rapidly than it had descended, and then descends 

 again less rapidly. All this agrees with the form given by theory. 



4. But though the general course of the curves has this resemblance, the amount 

 of flexure is not the same at different places. This result had already been obtained 

 by the comparison of previous observations, especially those made at Brest, Ply- 

 mouth, and London ; it is confirmed so clearly by the observations here referred to, 

 that I think it may now be assumed to be a general fact. 



The inferences from this fact are very important ; for in the first place it puts an 

 end to all attempts to deduce the mass of the moon from the phenomena of tides, or 

 to correct the tables of the tides by means of the mass of the moon. The approximate 

 agreement of the mass of the moon deduced by Laplace from the tides of Brest, with 

 the mass obtained from other phenomena, cannot be considered as otherwise than 

 accidental. If he had employed the tides of London, he would have obtained a mass 

 very different, as Mr. Lubbock has shown ; if he had taken those of Plymouth, or of 

 Brighton, the mass would have been again very different. 



This evidence of the inapplicability of this part of the theory will not surprise any 

 one who recollects how remote the hypotheses of the theory are from the case of 

 nature. Such a theory may point out the general features of the phenomena, but 

 any assumption of the actual correctness of the magnitudes determined by means of 

 it is altogether gratuitous. The force of the moon determines the amount of the 

 semimenstrual inequality ; but probably we shall never be able to ascertain otherwise 

 than empirically, hy what rule this force, producing oscillations in an ocean of irre- 

 gular form and depth, as the actual ocean is, determines the semimenstrual inequality 

 at each point. 



5. But since the semimenstrual inequality is thus determined in general by the 

 force of the moon, and has a common form at different places, and yet is different in 

 amount (and in other circumstances) at each place, we may represent it by resolving 

 it into two parts, one of which shall be common to the whole ocean, or to a large 

 portion of it, and the other part shall be a smaller and local correction, also following 

 a cycle of half a month. 



