1880.] Lord Rayleigh, On sensitive flames. 17 



(4) Every straight line from the "csecus focus" of the para- 

 bola to a point on the curve being said to be parallel to the axis, 

 the idea of the concurrence of parallel lines at a point at infinity- 

 has at length been formed and announced. It is to be noticed 

 that the new doctrine of parallels is here presented in relation to 

 one plane, and not as springing out of the consideration of figures 

 in perspective in space. 



Taking into account also Kepler's Nova Stereometria we con- 

 clude that by his contributions to the doctrine of the infinite and 

 the infinitesimal and his firm grasp of the principle of continuity, 

 he is entitled to the foremost rank amongst the founders of the 

 modern geometry. 



November 8, 1880. 



Professor Newton, President, in the Chair. 



The following communications were made to the Society: 



(1) On a neiv arrangement for sensitive flames. By Lord Ray- 

 LEIGH, M.A., F.R.S., Professor of Experimental Physics. 



A jet of coal gas from a pin-hole burner rises vertically in the 

 interior of a cavity from which the air is excluded. It then 

 passes into a brass tube a few inches long, and on reaching the 

 top, burns in the open. The front wall of the cavity is formed of 

 a flexible membrane of tissue paper, through which external sounds 

 can reach the burner. 



The principle is the same as that of Barry's flame described by 

 Tyndall. In both cases the unignitecl part of the jet is the sen- 

 sitive agent, and the flame is only an indicator. Barry's flame 

 may be made very sensitive to sound, but it is open to the objec- 

 tion of liability to disturbance by the slightest draught. A few 

 years since Mr Ridout proposed to enclose the jet in a tube air- 

 tight at the bottom, and to ignite it only on arrival at the top of 

 this tube. In this case however external vibrations have very 

 imperfect access to the sensitive part of the jet, and when they 

 reach it they are of the wrong quality, having but little motion 

 transverse to the direction of the jet. The arrangement now ex- 

 hibited combines very satisfactorily sensitiveness to sound, and 

 insensitiveness to wind, and it requires no higher pressure than 

 that of ordinary gas-pipes. If the extreme of sensitiveness be 

 aimed at, the gas pressure must be adjusted until the jet is on the 

 point of flaring without sound. 



VOL. IV. pt. I. 2 



