8 SCIENCE PROGRESS 



the moon has an apparent acceleration of its mean motion 

 relative to the sun. The period of the principal periodic term 

 is about 240 years ; the length of this period renders it diffi- 

 cult to separate the term depending upon the acceleration 

 from the periodic term. The problem of determining the 

 acceleration is therefore to some extent indeterminate. The 

 most probable value has been discussed in some important 

 papers recently by Dr. J. K. Fotheringham, which will be 

 reviewed in these notes in the next issue of Science Progress. 

 From these, it appears that an acceleration of 10" per century 

 must be very near the truth. Of this an amount of 6"*i can 

 be explained by purely gravitational causes. 



Various explanations of the remainder of the secular 

 acceleration have been put forward from time to time, but 

 until recently there was no theory which could be regarded as 

 wholly satisfactory. In default of any more plausible ex- 

 planation, it has generally been accepted, though without 

 quantitative evidence, that the balance was due to tidal 

 friction. Some recent work by G. I. Taylor has placed this 

 theory on a firmer theoretical basis. In a paper entitled 

 " Tidal Friction in the Irish Sea " {Phil. Trans., A. 220, 1-33, 

 191 9) he has calculated, by two independent methods, the mean 

 rate of dissipation of energy by tidal currents in the Irish Sea 

 at spring tides. One estimate is derived from an expression 

 for the friction between a tidal current and the sea bottom ; 

 the other is based upon the rate at which energy enters the 

 Irish Sea through the north and south channels, and the rate 

 at which lunar attraction does work on the waters of the Irish 

 Sea. The respective values derived by these two very different 

 methods are 1,300 and 1,530 ergs per sq, cm, per second, an 

 agreement which lends support to the validity of the argu- 

 ment. From this result Taylor calculates in a paper, "Tidal 

 Friction and the Secular Acceleration of the Moon " {M.N., 

 R.A.S., 80, 308, 1920), that the mean rate of dissipation of 

 energy in the Irish Sea is about 3 x 10^^ ergs per second. Dr. 

 H. Jeffreys has discussed the theoretical bearings of this theory 

 of tidal friction (" The Chief Cause of the Lunar Secular 

 Acceleration," Af.N., R.A.S., 80, 309, 1920), and finds that 

 the rate of dissipation of energy necessary to account for the 

 unexplained part of the secular acceleration of the moon is 

 about 1*4 X 10^' ergs per second. Taylor therefore remarks 

 that, on his above estimates, the Irish Sea contributes Ath 

 part of the total dissipation. The Irish Sea covers only 

 TTT.Vxnjth part of the water area of the globe, but very little 

 tidal dissipation occurs in the open ocean ; the amount only 

 becomes appreciable in long bays and channels where the 

 water is relatively shallow. The number of these is very 



