150 



REPORT — 1870. 



British Channel and German Sea, as compared with channels tkrough which 

 the ocean affects Liverpool. 



74. The search for astronomical long-period tides has up to this time given 

 only negative results. In each case a genuine annual tide seems to be indi- 

 cated, but the negative results as to the lunar fortnightly (declinational) tide 

 and monthly (elliptic) tide forbid us to regard either the semiannual or the 

 annual as a truly astronomical tide. Both are probably due to meteorological 

 causes. The cause which I have previously suggested (§ 1 0), that is, " water 

 received into the sea by drainage and the melting of ice, and from the di- 

 rect fall of rain into it," would tend to raise or lower the mean level almost 

 simultaneously over the whole sea. There are two other meteorological 

 causes which probably have very sensible effects, — difference in distribution 

 of atmospheric pressure over the earth, and difference of temperature in dif- 

 ferent oceans. These three causes may be sufScieut to explain the results of 

 observation collected in the following Table : — 



75. In conclusion, it may be remarked that sailors find nearly all they 

 want about the tides in British and Irish ports in the Admiralt)"- Tide-tables, 

 and the plan upon which they are constructed is available to give practical 

 results of similar value for most of the Atlantic Coasts. But this plan being 

 adopted solely for lunar and solar semidiurnal tides, is absolutely unavailable 

 to give any approach to good practical results for any ocean other than 

 the Atlantic, as in all other oceans the diurnal tides arc very considerable, 

 and in many localities are greater than the semidiurnal tides. 



7G. There exist but few records of tidal observations on coasts where the 

 diurnal tide is of this importance. Among those which do exist, however, 

 are some made at Bombay and Kurrachee, which have been treated by Mr. 

 Boberts and referred to in the foregoing Report. 



77. Former observations at these two ports had previously been analyzed 

 by Mr. Parkes, a member of this Committee, by a process described by him in a 

 paper presented to the Boyal Society, and published in the Philosophical Trans- 

 actions for 1868, and which he has since amended in some of its details. Mr. 

 Parkes has established formulae based on astronomical data, which represent 

 with considerable accuracy the movements of the diurnal tide ; and Tables 

 of the time and height of high and low water for the ports of Bombay and 

 Xurrachee have been computed under his direction, and issued under the 

 authority of the Secretary of State for India, for 1867 and each succeeding 

 year. This constitutes undoubtedly the first successful comparison of theory 

 and observation leading to sufiiciont tide-tables for any other than North- 

 Atlantic ports. If observations were made at other points on the coast of 

 India, and treated in a similar manner, further tables misjht be computedj, 



