On Objections to the received principles of Hydrodynamics. 159 



system of electrical measurement by which, from the simple de- 

 flection of a galvanometer-needle, we are able to predict how much 

 an electromagnet will sustain without actually trying the expe-, 

 riment. 



In looking over the columns of Table VIII. which contain the 

 values of the constants in the formula, we see how futile it is to 

 attempt to give any fixed value to the permeability of iron or 

 nickel ; and we also see of how little value experiments on any one 

 kind of iron are. Iron differs as much in magnetic permeability 

 as copper does in electric conductivity. 



It is seen that in the three cases when iron bars have been 

 used, the value of a is negative ; we might consider this to be 

 a general law, if I did not possess a ring which also gives 

 this negative. All these bars had a length of at least 120 

 times their diameter. 



The mathematical theory of magnetism has always been con- 

 sidered one of the most difficult of subjects, even when, as 

 heretofore^ fi is considered to be a constant; but now, when 

 it must be taken as a function of the magnetism, the diffi- 

 culty is increased many fold. There are certain cases, how- 

 ever, where the magnetism of the body is uniform, which will 

 not be affected. 

 Troy, June 2, 1873. 



XV. On Objections recently made to the received principles of 

 Hydrodynamics. By Professor Challis, M.A., LL.D., 

 F.R.S., F.R.A.S* 



A LARGE proportion of my scientific labours having been 

 devoted to researches respecting the principles and appli- 

 cations of Hydrodynamics, I have naturally taken much interest 

 in the discussion carried on in this Magazine between Mr. Moon 

 and the Hon. J. W. Strutt (Lord Rayleigh) relative to points 

 affecting the very foundations of that department of science. 

 After considering the arguments on both sides, it appears to me 

 that the questions raised demand additional elucidation, to sup- 

 ply which is the purpose of this communication. 



Mr. Moon's equation (1) in page 117 of vol. xxxvi. of the 

 Philosophical Magazine, viz. 



df + D dx 



includes the principle of constancy of mass, as is shown in the 



Note in p. 101 of vol. xlv., and for motion in one dimension is 



perfectly general. Respecting that equation Mr. Moon remarks 



* Communicated by the Author, 



