LAMPS. 



above the level of the wick, as in the old English ring-lamp called 

 the Sinumbral lamp, and a variety of other forms constructed on 

 me like principle. 



The flame issuing from such a wick is obviously a hollow 

 cylinder, and requires to be fed with air, both at its exterior 

 and interior surfaces. A current of air in contact with the 

 interior surface of the flame is maintained by carrying the 

 lesser of the two tubes between which the wick is included, 

 down through the burner, and leaving it in communication 

 with the external air. The exterior of the flame is exposed to 

 the air and produces currents by its own heat, in the same 

 manner as the currents already described, surrounding the flame 

 of a candle. 



But in the case of lamps with cylindrical burners these currents, 

 both exterior and interior, are greatly augmented in intensity 

 by the addition of a cylindrical glass- chimney of considerable 

 height, the inner diameter of which a little exceeds the exterior 

 diameter of the wick. This Chimney being open at its base, and 

 confining a column of air of its own height, acts upon the combustion 

 of the lamp exactly as a common chimney acts on the combustion 

 of fuel in a grate. The air which enters at the bottom, between this 

 chimney and the burner, rises in a cylindrical current around the 

 exterior of the wick, and passing in contact with the exterior surface 

 of the combustible vapour proceeding from the oil, ignites it at that 

 surface. The column of air which ascends at the same time 

 through the inner tube passing in contact with the inner surface 

 of the vapour ignites it in like manner. ' In this manner, a thin 

 cylinder of oily vapour rising from the wick is kept in a state of 

 vivid and constant combustion, both on its interior and exterior 

 surfaces. 



The force of these currents, exterior and interior, depends on the 

 buoyancy of the column of air included in the chimney, and which 

 also extends to a considerable height above it. The air after pas- 

 sing the flame of the lamp, being at a very high temperature, 

 the glass-chimney itself becomes intensely hot. The column of 

 air within the chimney being thus heated, it ascends to a consider- 

 able height above the chimney before it is cooled down to the tem- 

 perature of the surrounding air. The force of the draught which 

 maintains the currents around the flame is then determined by the 

 difference between the weight of the column of air, extending 

 from the base of the chimney to that height above it, at which the 

 temperature of the ascending column becomes equal to that of the 

 external air, and the weight of an equal volume of the external 

 air. 



This explanation of the combustion of the oil in a cylindrical 



207 



