20 EIGHTH REPORT — 1838. 



5. How does a change of the epoch affect the parallax correc- 

 tion of the heights ? 



6. How does a change of the epoch affect the declination 

 correction of the heights ? 



7. Does the parallax correction of the heights vary as the pa- 

 rallax ? 



8. Does the parallax correction of the times vary as the pa- 

 rallax ? 



9. Does the declination correction of the heights vary as 

 the square of the declination ? 



10. Does the declination correction of the times vary as the 

 square of the declination ? 



11. Can the laws of the corrections be deduced from a single year? 



12. Are there any regular differences between the corrections 

 of successive years ? 



13- Do the corrections of different places agree in laws and 

 amount ? 



The epoch here spoken of is that transit of the moon, anterior 

 to the tide, and to which the tide is referred. The question ex- 

 amined is, whether we obtain the closest accordance with the 

 observations by taking a transit one day, one and a half day, or 

 two days anterior to the tide which we consider. 



Although I have given the answers to these questions in the 

 memoir in the P/iilosopkical Transactions already referred to, I 

 here lay before the Association the curves*, the comparison of 

 which exhibits these answers, and exhibits indeed the result of 

 my discussions more clearly and exactly than words can do. 



The careful examination to which we have subjected the Bristol 

 tides, has shown us that there are scarcely any irregularities in 

 these phsenomena which m'c have not reduced, or may not hope to 

 reduce, to empirical laws, which laws constitute the first step to 

 the solution of our great tidological problem, the explanation of 

 the phfenomena on hydrodynamical principles. I may add that 

 the Report on Waves by Sir John Robison and Mr. Russell, inclu- 

 ded in the reports of the seventh meeting of the Association, con- 

 tains highly valuable materials, likely to assist us in the further 

 prosecution of this subject. The unexplained residue, which, in 

 our method of discussion, exhibits the difference between obser- 

 vation and our tables as hitherto corrected, although it is small 

 (upon the average two or three minutes in time, and as many 

 inches in height in a tide of forty feet), is so far seemingly subject 

 to some rule as to offer a promise of additional laws of cor- 

 rection, and I should be desirous of discussing this residual 

 quantity with such an object. 



* These curves are given in Plates 3, 4, 5, 6, 7, 8. 



