296 



Lord Rayleigh on the Production 



direction, so that a tongue o£ flame from a suitably placed 

 candle appeared to enter the nipple at the same time that 

 another candle situated immediately in front was blown away. 

 The two effects are oF course in reality alternating, and onlv 



Fig. 1. 



appear to be simultaneous in conse- 

 quence of the inability of the eye to 

 follow such rapid changes." 



The application of the principle 

 for the present purpose is very 

 simple. The candle is replaced by a 

 U pressure-gauge (fig. 1), the jet from 

 a contracted nozzle playing into one 

 limb. The whole is inclosed air- 

 tight in a test-tube, so that no per- 

 manent pressure or suction has any 

 effect upon the gauge. The nozzle 

 communicates by means of a flexible 

 tube with the space where the vibra- 

 tion is to be measured. Some throt- 

 tling to check the vibration of the 

 liquid may be convenient. 



The small Holmes apparatus gave 

 an indication of about 3 inches of 

 water, and the Trinity House 

 " Manual ^' one inch of mercury. 

 The sensitiveness may be lessened 

 by contracting the nozzle or probably 

 by insertion of water to diminish the 

 air-space within the test-tube. 



Reeds. 



Keeds have the advantage of working without a governor, 

 and the pitch once properly fixed is more likely to be main- 

 tained. I do not know whether reeds have been tried for 

 very large scale instruments. In the Barker apparatus* 

 three reeds are combined with one trumpet. At first sight it 

 may seem doubtful whether the tongues would vibrate in the 

 same phase, but upon examination I think it will appear that 

 this is the only way in which they could vibrate. On a 

 large scale either the reeds must be multiplied, or an entirely 

 different shape must be adopted, out of all proportion broader 

 than at present. Some experiments that I have tried seem to 

 show that the latter alternative is not impracticable. 



* Report of Trinity House Fog--Signal Committee on Experiments 

 conducted at St. Catherine's Point, Isle of Wight, 1901. 



