44 THE REV. W. WHEWELL ON THE EMPIRICAL LAWS 



closely with the theory, and which in its amount appears to depend only on the ratio 

 of the forces of the sun and moon, we find that in fact its amount is different at dif- 

 ferent places, as we have already stated. We cannot expect, therefore, that the 

 amount of the corrections for parallax and declination will agree very exactly with 

 those from theory; and till the empirical corrections are more certainly and generally 

 determined, I have not thought it worth while to make the comparison. 



But though there is at present this uncertainty respecting the amount of the in- 

 equalities of the tide, I do not conceive that there can be any doubt that the forms of 

 these corrections are such as I have stated them. In the case of the times of high 

 water especially, the general course of the variations of the quantities is as regular as 

 can be expected, and as is requisite for the establishment of our formulse. The 

 heights are much more anomalous ; probably they are more affected by winds, &c., 

 than the times are : and when we reflect that the tide at London may be affected by 

 the operation of causes in a remote part of the ocean, propagating their effect by the 

 progression of the tide-wave, we shall not be surprised at considerable deviations from 

 rule. The trade winds and other winds of the tropical regions may be felt in our tides, 

 and may even affect the means of long series of observations ; for it is to be recollected 

 that the averages which we obtain are not the averages of the effects of the sun and 

 moon alone, but the averages of their effect, together with that of meteorological 

 causes ; and it is very conceivable that the latter average may not vanish in the long 

 run. It is moreover to be observed, that the peculiar circumstances of London, in 

 having a tide compounded of two tides, arriving by different roads after journeys of 

 different lengths, may easily be supposed to give rise to additional chances of irregu- 

 larity. 



It may not be superfluous to remark, that, independently of such a combination of 

 circumstances, there is nothing in the situation of the port of London to diminish the 

 value of tide observations there. The length and windings of the river by which the 

 tide reaches the port present no objection to the comparison of the observations with 

 theory. These circumstances may modify the tide, but they modify it alike every 

 day, or at least alike at like periods of the tidal cycles, and therefore they introduce 

 no irregularity. Indeed, there are some reasons for believing that the tides in rivers 

 and deep sounds are more regular than those on the open coast ; and at any rate, as 

 they are generally larger in such situations, their variations are more observable. 



Concluding Observations. 



It appears from the preceding investigations and considerations, that the following 

 are now the most important steps from which any great improvement of our know- 

 ledge on this subject may be hoped. 



Large collections of observations at other places must be discussed in a manner 

 resembling that employed for the London observations by Mr. Dessiou. The Brest 

 and the Liverpool observations would be excellent materials for such operations. We 



I 



