480 bepokt— 1878. 



perceptible to ordinary observation, and nothing, therefore, has been hitherto gene- 

 rally known regarding their character. But a first case of application of the 

 harmonic analysis to the accurate continuous register of a self-recording tide-gauge 

 (published in the 1876 report of the B.A. Tidal Committee) has shown for Toulon a 

 diurnal tide amounting on an average of ordinary midsummer and mid-winter full 



and new moons to nearly — of the semi-diurnal tides ; and the present commu- 



o 



nication contains the results of analysis showing a similar result for Marseilles ; but, 



on the other hand, for Malta, a diurnal tide (similarly reckoned), amounting to only 



— of the semi-diurnal tide. The amount of semi-diurnal tide is nearly the same in 



4 £ . . 



the three places, being, at full and new moon, about seven inches rise and fall. 



The present investigation commenced in the Tidal Department of the Hydro- 

 graphic Office, under the charge of Staff-Commander Harris, R.N., with an exami- 

 nation and careful practical analysis of a case greatly complicated by the diurnal 

 inequality presented by tidal observations which had been made at Freemantle, 

 Western Australia, in 1873-74, chiefly by Staff-Commander Archdeacon, It.N., the 

 officer in charge of the Admiralty Survey of that Colony. The results disclosed 

 very remarkable complications, the diurnal tides predominating over the semi- 

 diurnal tides at some seasons of month and year, and at others almost disappearing 

 and leaving only a small semi-diurnal tide of less than a foot rise and fall. These 

 observations were also very interesting in respect to the great differences of mean 

 level which they showed for different times of year, so great that the low-waters 

 in March and April were generally higher than the high-waters in September and 

 October. The observations were afterwards, under the direction of Captain Evans 

 and Sir William Thomson, submitted to a complete harmonic analysis, worked out 

 by Mr. E. Roberts. Not only on account of the interesting features presented by 

 this first case of analysis of tides of the southern hemisphere, but because the 

 south circumpolar ocean has been looked to, on theoretical grounds, as the origin of 

 the tides, or of a large part of the tides, of the rest of the world, it seemed 

 desirable to extend the investigation to other places of the southern hemisphere for 

 which there are available data. Accordingly the records in the Hydrographic Office 

 of tidal observations from all parts of the world were searched ; but besides those of 

 Freemantle, nothing from the southern hemisphere was found sufficiently complete 

 for the harmonic analysis except a year's observations of self-registering tide-gauge 

 at Port Louis, Mauritius, and personal observations made at regular hourly, and 

 sometimes half-hourly, intervals for about six months (May to December) of 1842, 

 at Port Louis, Berkeley Sound, East Falkland, under the direction of Sir James 

 Clark Ross. These have been subjected to complete analysis. 



So also have twelve months' observations by a self-registering tide-gauge 

 during 1871-72 at Malta, contributed by Admiral Sir A. Cooper Key, K.C.B., 

 F.R.S. 



Tide-curves for two more years of Toulon (1847 and 1848), in addition to the 

 one (1853), and for Marseilles for a twelvemonth of 1850-51, supplied by the 

 French Hydrographic Office, have also been subjected to the harmonic analysis. 



These results, both for the southern hemisphere and the Mediterranean, will 

 form the subject of a paper which Captain Evans and Sir William Thomson hope 

 to communicate to the Royal Society in the course of the coming session. In the 

 meantime the numbers resulting from the harmonic analysis are submitted without 

 further comments to the British Association for comparison with those for other 

 places in previous Reports of the Tidal Committee. Those of them which represent 

 the most important of the diurnal and semi-diurnal tides are shown in the following 

 table, which includes also for immediate comparison the results for Toulon, 1853. 



It in every case denotes, as in previous tables of the British Association Com- 

 mittee, the range of the particular tidal constituent on either side of mean level ; 

 so that 2-R is the whole rise from lowest to highest of the individual constituent. 

 (In comparing results with those shown in the Admiralty Tide Tables, it must be 

 borne in mind that in the latter it is the rise above the level of ordinary low water 

 spring tides that is given as " heights." 



