18 Fishery Board for Scotland. 



Of observations upon the annual variation of Mean Sea Level in 

 regions remote from our own seas, I can only quote the case of Aden, 

 which has been frequently dealt with, for instance, by Sir George 

 Darwin,* and by Professor Kriimmel.t 



Table !.•— Variation of Mean Sea Level at Aden (in feet). 



Here we have again a very simple curve, which may be approxi- 

 mately expressed (to the epoch of mid- January) as A + -376 cos 

 (t-62°) + -lllcos(2t + 98°). 



It will be noticed that the phase is nearly the opposite of that 

 which we have been dealing with in British Seas, the maximum being 

 now in spring and the minimum in late summer. 



As regards the causation of these annual and semi-annual tides, 

 it is generally held that, as Lord Kelvin said, they are not true tides 

 of astronomical origin, but are due to meteorological causes. The solar 

 tides of annual and semi-annual period are, as Sir George Darwin 

 reminds us, J " probably quite insensible as arising from astronomical 

 causes." The annual tide is of quite microscopic minuteness, and even 

 the semi-annual tide, which is many times larger, is very small. 

 Yet we have seen that the actual annual and semi-annual tides are of 

 very considerable magnitude and regularity, and are taken careful 

 account of, wherever possible, in tide-prediction. Whatever the 

 cause of these fluctuations may be, it is commonly stated that it 

 must be of terrestrial, or " meteorological " origin. But as to the 

 direct raison d'etre of these tides. Sir George Darwin, writing to me 

 a few years ago (1906), said : " It seems to me unsafe to speculate 

 as to how far an annual inequality may be due to ice-melting, 

 rainfall, and acceleration of ocean currents. At least I would not 

 hazard a conjecture." 



There is little which I can contribute towards the solution of 

 the problem. Let me try, however, to describe, very briefly, the 

 work which has already been done. 



Of the meteorological causes which have been suggested in order 

 to account for our phenomena, there are some which seem to have very 

 little or no appreciable significance. Among these I would place 

 rainfall and evaporation, and the effects of frost and thaw. On our 

 East Coast of Scotland, at least, the annual curve of rainfall is very 

 much more irregular than is the curve of sea level which we desire 

 to explain ; nor do the periods of maximum and minimum by any 

 means agree. We have still to seek for phenomena which shall explain 

 the great similarity in character of the fluctuations in sea level that we 

 have found in such different regions as Milford and Dundee, the 

 Dutch Coast and the Baltic Stations ; regions in which we have very 

 wide variations in climate, in rainfall, in the magnitude of the winter 

 frosts, in the proximity of great rivers, in short in very many of the 



* Scientific Papers, pp. 300, 301. 



f Of. cit., i. p. 62. The data are taken from the Verhandl. d. 12 allg. Konf. 

 d. intern. Erdmessung, 1898, Stuttgart, p. 379, 

 X Op. cit., vol. i. p. 28. 



