LAWS OF THE TIDES FROM SHORT SERIES OF OBSERVATIONS. 



245 



It appears from these Tables that the greatest difference of the semimenstrual in- 

 equality of time in any one year from the mean of the four years is 4j minutes ; and 

 the greatest difference of height in the same manner is about 4 inches. It is to be 

 recollected that this is the error in a mean tide of 33 feet. 



XIII. Do the corrections at different places agree in laws and amount ? 



The amount of agreement in this respect will be seen by the following statement. 



The semimenstrual inequality in time, although according to theory it is deter- 

 mined by the proportion of the force of the moon to that of the sun, is by the obser- 

 vations different at different places. The total amount of this inequality, that is, the 

 difference of the greatest and least lunitidal intervals, is 95 minutes at Plymouth, and 

 93 minutes at Bristol. But these are greater than the values which this inequality 

 assumes at any other places. It is 90'" at London and at Sheerness ; 86"* at Liver- 

 pool and Howth ; 84™ at Leith ; 83"* at Portsmouth and at Pembroke ; 82°* at Rams- 

 gate ; and only 80"* at Brest. In each of these cases the value is determined from 

 observations so numerous as to be certain within a minute or two. We see, therefore, 

 how different the mass of the moon would be found to be by calculations from the 

 tide observations of different places. 



The comparison of the semimenstrual inequality of height at different places gives 

 a similar result. In this case, however, we must not take the actual amount of the 

 inequality, but its proportion to the mean tide. At Portsmouth the mean range of 

 the tide is 12J feet ; the total semimenstrual inequality, or difference of the height of 

 high water at neap and spring tides, is 2^ feet ; that is, only ^ of the mean tide. But 

 at Plymouth, where the mean tide is also 12 J feet, the total semimenstrual inequality 

 is 3'4 feet, and thus the fraction is 3^. And at Bristol, where the mean range of tide 

 is 33 feet, the semimenstrual inequality is 10 feet, or ^ of the mean tide. 



