On the Dimensions of a Magnetic Pole. 225 



substance is perfectly -without friction, the fortnightly ocean- 

 tide in an equatorial canal would be reduced to about 0"6 of its 

 calculated amount. It would require a coefficient of friction 

 of O01 to bring this tide up to 0'7 of its calculated value. But 

 the same amount of friction would have so great an effect in 

 reducing the semidiurnal tide in the substratum, that the 

 ocean -tide of that period would not be perceptibly affected. 

 This is entirely in accordance with the a priori reasoning sug- 

 gested in my book, as referred to at the beginning of this 

 article. Without friction, however (a condition impossible in 

 nature), the semidiurnal tide would be reduced to one half its 

 calculated value. 



Should then observations on the fortnightly tide lead to the 

 conclusion that it is reduced to somewhere about 07 of its cal- 

 culated value, it would appear that such a result would on the 

 canal theory agree perfectly well with the theory of a liquid 

 substratum upon a rigid nucleus. And, further, under these 

 circumstances no appreciable diminution of the semidiurnal 

 ocean-tide could be expected. 



The entire range of the fortnightly tide at Teneriffe, upon 

 the supposition of a rigid earth, would be, according to Sir 

 William Thomson, 4'5 inches*. 

 August 12, 1882. 



XXY. On the Dimensions of a Magnetic Pole in the Electro- 

 static System of Units. By J. J. Thomson* 



To the Editors of the Philosophical Magazine and Journal. 



Trinity College, Cambridge, 



Gentlemen, August 21st, 1882. 



IIXFEE from Prof. Clausius's letter in the last ISTumber 

 of the Philosophical Magazine that he has misunderstood 

 my position with regard to the question of the dimensions of 

 a magnetic pole in the electrostatic system of units. I did 

 not attempt to show that Maxwell's value was in accordance 

 with Ampere's theory as Prof. Clausing interprets it, but en- 

 deavoured to show that Maxwell's value was the necessary 

 consequence of the principles laid down in his treatise ; and 

 to point out what modification of Ampere's theory these prin- 

 ciples lead to. From what Prof. Clausius says about Ampere's 

 theory being independent of electrodynamic considerations, 

 it would seem that he understands the theory to state that every 

 small magnet is an electric current, and not that the magnetic 

 effects of every small magnet may be represented by those of 

 * Natural Philosophy, § 845, ed. 1867. 



Phil. Mag. S. 5. Vol. 14. No*. 87. Sept. 1882. Q 



