TIDES, SOUTH AUSTRALIA. 231 



stated in conformity with their notation. Owing, un- 

 fortunately, to the smallness of the scale of the gauge (3 feet 

 to 1 inch), it seemed useless to analyse for the smaller short 

 period tides, but probably all those which have any appreciable 

 effect have been investigated. The tidal curve for 12 months 

 was taken for analysis, commencing midnight February 28th, 

 1889. As usual, there were a few gaps in the record owing 

 to the breaking down of the tide-gauge at intervals, but 

 these were for the most part short, and fortunately we were 

 able to fill these in from records kept at the Semaphore, close 

 by, of the times and heights of high and low water. The 

 datum-level is the same at both places, and, as the times and 

 heights observed here generally agree very closely with those 

 obtained with the tide-gauge, the missing portions of the 

 curves could be traced in from these observations and the 

 gaps supplied with considei'able accuracy. The numerical 

 I'esults of the Harmonic Analysis are given in the table 

 annexed. The lunar and solar semi-diurnal tides have the 

 greatest amphtudes (about 3 feet total range), and are almost 

 of exactly equal value. Sir W. Thomson also found that at 

 Fremantle these two tides are equal, but thei-e their ampli- 

 tudes are much less than at Adelaide. The declinational tides 

 are comparatively large, the principal diurnal having a total 

 range of 1"67 feet. Evidently these facts give the explanation 

 of the " dodging tides," for near the neaps the lunar and 

 solar semi-diurnals almost destroy one anothei', and the 

 declinational tides predominate over them. Near the 

 equinoxes, however, when the declinational tides almost 

 vanish, the complete interference of the two principal semi- 

 diurnal tides causes the level of the water to remain almost 

 perfectly constant for perhaps 24 hours, and the nature of 

 the curve at this time is exactly that caused by the inter- 

 ference of two harmonic components. 



The principal solar semi-diurnal tide gives, at Adelaide, 

 high-water exactly at 6 o'clock, low-water exactly at 12. 



