214 Rev. 0. Fisher on the Effect upon the Ocean-tides 



denser substratum of molten rock, the elevations on the sur- 

 face of the crust being due to compression, and being sup- 

 ported through flotation by corresponding protuberances (which 

 I call "roots of the mountains ") projecting downwards into 

 the denser liquid — a mode of support long ago suggested by 

 Sir a. B. Airy*. 



The most formidable difficulty in the way of this theory has 

 been said to be the necessary occurrence of tides in such a 

 substratum; and it has been thought that, the crust being 

 carried up and down in sympathy with the substratum, the 

 ocean-tides would be almost entirely masked, and that there 

 would be no appreciable rise and fall of the water relatively 

 to landf. 



The explanation of the difficulty which I had offered in the 

 book itself was, that the tides in the substratum would involve 

 a horizontal transference of fluid backwards and forwards, 

 and might be expected to be of small amplitude, owing to the 

 viscosity of the substance and to its confinement beneath the 

 crust J; and I felt so convinced, from geological considera- 

 tions, that the substratum must be at least plastic, if not liquid, 

 that I did not think it needful to go further into the question. 

 But in consequence of the weight of authority by which this 

 objection has since been enforced, I have been induced to 

 examine it more closely, and to endeavour to discover what 

 indications, if any, the ocean-tides might be expected to give 

 of the existence, or otherwise, of such a substratum. 



(2) The " canal theory " of the tides appears to be the 

 most suitable to solve, in a general way, the question at issue; 

 for what we have to do is to investigate the motion of layers 

 of liquids (the substratum and the ocean)under the influence 

 of tide-producing forces. 



An article upon the tides by Mr. D. D. Heath appeared 

 in this Magazine § in 1867. He intimates that it was founded 

 upon Airy's treatise in the Encyclopaedia Metropolitana. I 

 shall take the liberty of adopting the introductory paragraphs 

 of his analysis, merely changing a symbol. The liquid is 

 treated as " confined to a narrow channel running round the 

 equator, supposing the moon vertical over it and moving uni- 

 formly in her orbit, so that her apparent motion will be also 

 uniform and somewhat less than that of the earth's rotation." 



" § 3. And first as to the geometrical characteristics of a 

 fluid wave uniformly propagated westward at any rate (a). 



* Phil. Trans. Roy. Soc. vol. cxlv. p. 101. 



t ' Nature,' vol. xxv. p. 423, 1882 ; also New- York ' Nation/ June 15, 

 1882. 



\ < Physics of the Earth's Crust,' p. 23. 



§ Fourth Series, vol. xxxiii. p. 165, March 1607. 



