1837.] the South African Literary and Scientific Institution, 397 



with all the greater phenomena of meteorology, assumes a high degree 

 of interest, and will no doubt be studied with the perseverance and ex- 

 actness it merits. 



A series of observations of the heights and times of high and low 

 water at Simon's Bay, extending from January 26, to June 30, has been 

 obligingly submitted to the consideration of the Meteorological Com- 

 mittee, by J. Deas Thomson, Esq. and the Astronomer-Royal. It has 

 not yet been possible to compare them with any theory, and indeed it 

 would be premature to attempt it here, as they will require to be com- 

 bined with the mass of knowledge now accumulating on this subject 

 in Europe, to render them in any degree available. One remarkable 

 result, however, may be mentioned here, which offers itself on a very 

 cursory inspection of the heights, as compared with the declinations 

 of the Sun and Moon, viz. that w 7 hile the monthly fluctuation of the 

 mean sea-level, arising from the moon's alternate occupation of the 

 northern and southern hemisphere, is scarcely perceptible, amounting 

 hardly to two inches, its annual variation, due to the similar approach 

 of the sun to the northern and southern solstice, is much more consi- 

 derable, and forms indeed a prominent feature in the Tides of this coast, 

 amounting to no less than eight inches, or nearly a fifth of the average 

 difference between high and low water — as the follo'wing brief Table 

 will shew — in which the interval embraced by the observations is 

 divided, not as usual into lunations from full to full or from new to new 

 Moon, but into periods marked by the moon's passing from south to 

 north of the' equinoctial. By this division the effect (if any) of the 

 moon's change of declination compensates itself, and leaves the solar 

 effect in evidence. The cause of the prominence thus given to this 

 part of the sun's agency, appears to lie in the length of its period com- 

 pared with the moon's, which gives time for the waters of the whole 

 ocean to accommodate their general level to the actual force, by bodily 

 transfer from one part of the globe to another, and by assuming, at each 

 instant (what the tides of short period have never time to do), very 

 nearly the figure of equilibrium due to this particular modification of 

 the disturbing forces. 



Observed Mean Positions of the Mid-water mark on the Float of the 

 Tide-gauge at Simon's Bay, during successive intervals of the Moon's 

 Transit from North to South of the Equinoctial. 







No. of 



H 



sights of Mid- 





Limits of intervals 





Tides 





water on the 



Remarks. 







observed. 





Gauge. 



. Jan. 26 to Feb. 



16 



22 

 27 



4 feet 4.38 in. 



period incomplete. 



Feb. 17 to Mar. 



15 



4 



.. 2.35 .. 



Mar. 16 to April 



11 



27 



3 



.. 10.83 .. 





April 12 to May 



9 



28 



4 



.. 0.22 .. 





May 10 to June 



5 



27 



3 



.. 9.52 .. 



period incomplete, j 



1 June 6 to June 



30 



25 





.. 8.37 .. 





