TIDAL OBSERVATIONS. 123 



The Exeter Report coueludes with remarks on : — 



I. Eetardation of times of spring-tides after new or full moon, and 

 deductions as to retardation of earth's rotation. 

 II. Diurnal tides (or constituents having approximately 24:^ for period). 



I. Times of Spring-tides after Full Moon ami New Moon. 



50. Dr. Thomas Young gave earlier than Airy, and probably first of all, 

 the dynamical theory that retardation of times of spring-tides after the times 

 of full and change implies friction. The results now presented by the Com- 

 mittee verify the anticipation that there is much retardation in every sea. 

 A few more years of patient work at harmonic rediiction, and some sets of 

 good observations from places in the China seas, Antarctic sea, and Pacific, 

 will afford means of directly estimating the loss of energy from the earth's 

 rotation, and will confirm the evidence which Professor Huxley and the 

 geologists, for whom ho speaks, find so hard to accept, that energy is being 

 dissipated too rapidly to leave credible any thing approaching to so great 

 drafts on time as they have been accustomed to make. 



II. Diurnal Tides. 



51. A not hitherto explained characteristic of North-Atlantic stations is 

 absence of diurnal tides large enough to be discovered except by scientific 

 analysis. The diurnal components are very conspicuous in the Bombay and 

 Fort-Point tides, especially the Fort Point. That one of the components of 

 lunar and solar diurnal tides whose argument is yt, is very large for Fort 

 Point. Hence, considering that that component, being partly due to moon 

 and partly to sun, gives a thoroiujlily true theoretical method for comparing the 

 sun's and moon's masses, by using a series of nine years' observations, it wiU 

 in all probability give a somewhat accurate practical result. As to the mag- 

 nitudes of the diurnal tides in different localities, it is to be remarked that 

 their smallness on the North- Atlantic coasts is irregular, and has not yet been 

 explained dynamically. Their largeness in the Indian Ocean, the China 

 seas, and the Pacific is regular, but makes the tidal phenomena much more 

 complicated than those we know best. The tides in those seas are commonly 

 designated as " irregular." That designation results from a confusion of 

 terms, " irregular" being used as if synonymous with complicated. The 

 truth is that the tides on the European coasts of the Atlantic are irregularly 

 simple ; those in aU other seas are comparatively complicated, but regular 

 and explicable. 



[Conclusion of Exeter Report, dated from Largs, Ayrshire, Aug. 21, 1869.] 



Jlejiort by Sir William Thomson, ivith detailed Statement 6?/Mr. E. Roberts, 

 of the work performed by him for the Committee since the Meeting at 

 Exeter. 



The College, Glasgow, Sept. 10, 1870. 



52. The work performed since the Meeting at Exeter has been mainly 

 directed towards a full scientific analysis of the tides of Liverpool ; but it has 

 included also as much as could be reached in the way of analyzing observa- 

 tions on the tides of Kurrachee supplied by Mr. Parkes, and on the tides of 

 Fort Point, California, supplied by the United States' Survey. With reference 

 to the latter, I have just received the follovdng interesting letter from Mr. 



70) of the Eeport presented to the Liverpool Meeting and now printed in continuation of 

 the Exeter Beport. - 



