ACTUAL STATE OF EAETH. 35 



the sea begins sensibly to diminish in approaching the 

 coast, but this we are unable to do for want of data.) 



These instances are sufficient to show the difficulty (per- 

 haps amounting to impossibility) of determining whether 

 there is any preponderance of retardation at all. At all 

 events, however, it is clear that the retardation, if any, 

 must fall very far short of the maximum. 



It is to be remembered, further, that in the case con- 

 sidered above of a globe uniformly covered with water, 

 each section of the globe parallel to the equator has its 

 own tidal current to encounter its own inertia, and hence 

 the result in the case of the equatorial canal was applicable 

 to the entire globe. But in the case of the earth it is 

 not so, and this would still further dimmish the retar- 

 dation. 



On the whole, it would appear that no certain conclusion 

 as to the amount of retardation of the earth's rotation by 

 the tides can be drawn from theoretic considerations. 



I am not aware that any estimate of the value of the 

 coefficient / has been published. The following has 

 been kindly communicated to me by the late Professor 

 Fitz Gerald: 



Let fj. = the coefficient of viscosity of water. 



Then x being the ordinate of the water, let 

 d z V dV 



(h being the depth in centimetres) 



(since tf> =fv) 





