IO 



NA TURE 



[July i, 1922 



Therefore, even after one complete sound wave there 

 will be already a clearly marked selection of the 

 " in-tune " group of resonators. 



Taking now the physiological effects into con- 

 sideration, the threshold factor will definitely rule 

 out all amplitudes below a certain value, so that 

 after a certain number of sound waves have entered 

 the ear the amplitude of the " in-tune " resonators 

 will be the first to rise above the threshold and will 

 cause impulses to pass up the auditory nerve. A 

 little later a larger number of resonators will have 

 reached amplitudes above the threshold, so that there 

 should be a gradual increase in the number of reson- 

 ators in active response, until the full steady value 

 is reached. Calculation shows that the "in-tune" 

 resonator should attain 90 per cent, of its final ampli- 

 tude in eight vibrations. On the resonance theory 

 one would therefore expect a gradual rise in the sound 

 intensity occurring in a time interval of the order of 

 10-20 vibrations of the incoming sound waves (i.e. 

 1/25-1/12 sec. for middle C), and not a fall as Mr. 

 Ackermann has suggested in his letter. It would 

 seem that this effect is responsible for the absence 

 of roughness at the commencement of a tone due 

 to the imperception of the transient vibrations of 

 " out-of-tune " resonators. 



Now if the rise of sound intensity is a gradual one, 

 what, it may be asked, is the mode of perception of a 

 tone which starts with large amplitude and gradually 

 diminishes as it goes on — e.g. a piano note ? In 

 such a case it would seem that after a very few 

 vibrations, the swings of the resonators must reach 

 such an amplitude that their motion is perceived. 

 In this case, then, the vibration of "out-of-tune" re- 

 sonators makes itself perceived because the auditory 

 nerve fibres are taking up responses before there have 

 been sufficient incoming sound waves to damp out 

 the " out-of-tune " resonators. It would seem to 

 be this effect which gives the transient harshness to 

 the commencement of a piano note, causing it to 

 sound to the ear as if it started with a consonant. 



H. Hartridge. 



King's College, Cambridge. 



An Experimental Towing-tank used by 

 Benjamin Franklin. 



In the " Calendar of Industrial Pioneers " in 

 Nature, May 6, p. 598, relative to the anniversary 

 of William Froude, your correspondent says: " His 

 (Froude) work led" to the construction by the 

 Admiralty of the experimental tank at Torquay, 

 the first of its kind ever built." 



It will be doubtless interesting to readers of 

 Nature to have it brought to their attention that 

 Benjamin Franklin 111 his many and varied investiga- 

 tions in philosophical subjects investigated, to some 

 extent, the difference of navigation in shoal and 

 deep water. In a letter written to Dr. John Pringle, 

 May 10, 1768, he gives the results of experiments 

 made along these lines. The letter tells of how, 

 during a trip with Dr. Pringle in Holland, it was 

 brought to their attention that the treckschuytm one 

 of its trips went slower than usual, due, as the 

 boatmen explained, to the water in the canal being 

 [< .w . After his return to England, not being entirely 

 satisfied with the boatman's explanation, Franklin 

 questioned the Thames river watermen and found 

 them all agreeing as to fact, but differing widely in 

 expressing the quantity of the difference. He, 

 therefore, designed the following experiment, which 

 in its nature is a forerunner of the modern towing- 

 tank. I quote from his letter : 



NO. 2748, VOL. I io] 



" I provided a trough of planed boards fourteen 

 feet long, six inches wide, and six inches deep in the 

 clear, filled with water within half an inch of the 

 edge, to represent a canal. I had a loose board, of 

 nearly the same length and breadth, that, being put 

 into the water, might be sunk to any depth, and 

 fixed by little wedges where I would choose to have 

 it stay, in order to make different depths of water, 

 leaving the surface at the same height with regard to 

 the sides of the trough. I had a little boat in form 

 of a lighter or boat of burden, six inches long, two 

 inches and a quarter wide, and one inch and a quarter 

 deep. When swimming, it drew one inch water. To 

 give motion to the boat, I fixed one end of a long silk 

 thread to its bow, just even with the water's edge ; 

 the other end passed over a well-made brass pulley 

 of about an inch diameter, turning freelv on a small 

 axis ; and a shilling was the weight. Then, placing 

 the boat at one end of the trough, the weight would 

 draw it through the water to the other. 



" Not having a watch that shows seconds, in 

 order to measure the time taken up by the boat in 

 passing from end to end, I counted as fast as I could 

 count to ten repeatedly, keeping an account of the 

 number of tens on my fingers. And as much as 

 possible to correct any little inequalities in my 

 counting, I repeated the experiment a number of 

 times at each depth of water, that I might take the 

 medium. And the following are the results : 



Paul C. 

 U.S. Coast and Geodetii Sui ve\ 

 \\ .Islington, D.C., May 22. 



An Experimental Confirmation of the Kinetic and 

 Molecular Theories of Magnetism. 



Curie's law states that ferromagnetics above 1 lie- 

 critical temperature behave in such a way that the 

 susceptibility (In is inversely proportional to the 

 absolute temperature (7"), in short, that the product 

 A- . T is a constant. The physical meaning of this 

 law is that when the molecular magnets have 

 complete freedom of rotational movements, the 

 energy of magnetisation is then only opposed by the 

 energy of thermal agitation and, consequently, any- 

 given state is a state of equilibrium. 



Below the critical temperature complications intro- 

 duced by the mutual magnetic actions of the mole- 

 cules, one on the other, and by the approach to a 

 saturation limit have obscured any such simple law. 

 It is, however, possible to eliminate, or allow for. the 

 effects of these disturbing factors and to make 

 experiments, under hysteresis-free conditions, upon 

 the variation of susceptibility with temperature. 

 Experiments of this kind have been carried out on 



