On "S" Radiation. 719 



It appears from experiment that cj£> is sensibly constant for 

 all large values of Dp$/fi; or, if the equation be written in 

 the equivalent form 



p W=Kd7s) = * (II) ' • • ■ (64) 



•^(11) is constant, so far as is known, down to the smallest 

 values of II that have been realized experimentally. And 

 yet Hydrodynamics tells us that if yu, = 0, and therefore 

 11 = 0, exactly, there is no resistance at all; so that in the 

 immediate vicinity of 11 = 0, ^(XI) must fall very rapidly 

 from a nearly constant finite value to zero. 



It is unlikely that a difficulty of this sort occurs to com- 

 plicate the problem of the spheroid, and it seems safe to 

 assume that Lord Rayleigh ; s solution would be sensibly 

 exact for a body of any ordinary liquid, even a very viscous 

 one, of the order of magnitude of the Earth. But the dis- 

 regarding of small quantities, while nearly always justified, 

 always involves an assumption and we must always be 

 prepared for failure. For no accuracy of logic can deduce 

 a true result from false premises. 



In concluding these notes, which are merely a series of 

 remarks that are obvious as soon as made, one final remark 

 may be added. It is, that what is needed for the successful 

 use of dimensional reasoning is neither algebra nor meta- 

 physics, but only practice and a very modest portion of 

 that physical common sense which the late Lord Eayleigh 

 exhibited in so eminent a degree. 



Bureau of Standards, 

 April 12, 1921. 



LXXX. " J" Radiation. By J. A, Orowther, M.A., Sc.D, 

 F.Inst.P., Cavendish Laboratory, Cambridge *. 



Introduction . 



THE question as to the possible existence of fluorescent 

 X radiations of a considerably shorter wave-length 

 than those of the well-known K series of radiations is one 

 which must have confronted all workers who have attempted 

 to make accurate experiments on the scattered radiation, 

 especially on that from elements of low atomic, weight. 



* Communicated by Professor Sir E. Rutherford, F.R.S. 

 The expenses of this research were partly defrayed by a grant from 

 the Government Grant Committee of the Royal Society. 



