﻿442 Lord Rayleigh on Molian T< 



ones. 



have nearly the full A r alue, say "190. The reciprocal o£ this, 

 or 5*3, should agree with the ratio found ahove as 7*0 ; and 

 the discrepancy is larger than it should be. 



An experiment to try whether a change of viscosity had 

 appreciable influence may be briefly mentioned. Observa- 

 tions were made upon water heated to about 60° C. and at 

 12° C. No difference of behaviour was detected. At 60° C. 

 ^=•0049, and at 12° C. yu,= '0124. 



I have described the simple pendulum apparatus in some 

 detail, as apart from any question of measurements it de- 

 monstrates easily the general principle that the vibrations 

 are transverse to the stream, and when in good action it 

 exhibits very well the double row of vortices as witnessed 

 by dimples upon the surface of the water. 



The discrepancy found between the number from water 

 (7*0) and that derived from StrouhaFs experiments on air 

 (5*3) raises the question whether the latter can be in error. 

 (So far as I know, Strouhal's work has not been repeated ; 

 but the error most to be feared, that arising from the circu- 

 lation of the air, acts in the wrong direction. In the hope 

 of further light I have remounted my apparatus of 1879. 

 The draught is obtaiued from a chimney. A structure of wood 

 and paper is fitted to the fireplace, which may prevent all 

 access of air to the chimney except through an elongated 

 horizontal aperture in the front (vertical) wall. The length 

 of the aperture is 26 inches (66 cm.), and the width 4 inches 

 (10*2 cm.) ; and along its middle a gut string is stretched 

 over bridges. 



The draught is regulated mainly by the amount of fire. 

 It is well to have a margin, as it is easy to shunt a part 

 through an aperture at the top of the enclosure; which can 

 be closed partially or almost wholly by a superposed card. 

 An adjustment can sometimes be got by opening a door or 

 window. A piece of paper thrown on the fire increases the 

 draught considerably for about half a minute. 



The string employed had a diameter of *95 mm., and it 

 could readily be made to vibrate (in 3 segments) in unison 

 with a fork of pitch 256. The octave, not difficult to mis- 

 take, was verified by a resonator brought up close to the 

 string. That the vibration is transverse to the wind is con- 

 firmed by the behaviour of the resonator, which goes out of 

 action when held symmetrically. The sound, as heard in 

 the open without assistance, was usually feeble, but became 

 loud when the ear was held close to the wooden frame. 

 The difficulty of the experiment is to determine the velocity 

 of the wind, where it acts upon the string. I have attempted 



