190 Domestic Science 



A small gas-jet at the end of a bent glass tube 

 attached to the ordinary gas supply, is lowered into 

 one of the wide tubes as illustrated. An upward 

 current of heated air ascends this tube and a current 

 of cool air passes downward in the other. The presence 

 of these streams of air may be rendered visible by hold- 

 ing a smouldering piece of soft brown wrapping paper 

 at the top of the second tube, when the course taken 

 by the smoke indicates the directions of the currents. 



This experiment is an illustration on a small scale 

 of a method used in ventilating coal-mines in which 

 there is no danger of explosion on account of the 

 presence of explosive mixtures of air with the gases 

 sometimes given off in such mines. A large fire is 

 built beneath the end of a deep shaft, known as the 

 " upcast " shaft, and fresh air passes down a similar 

 shaft, the " downcast " shaft, situated in the part of 

 the mine furthest from the former. This air is thus 

 obliged to traverse the various " workings " of the 

 mine in its passage from one shaft to the other and 

 the miners are enabled to continue at work without 

 harm from the fouling of the air by the products of 

 respiration. An ordinary room, in which a fire is 

 burning, is ventilated in a similar fashion. The heated 

 air goes up the chimney and fresh air enters through 

 the apertures in the walls of the room, such as window 

 and door openings and the cracks between the fittings 

 of the woodwork, as well as through the porous brick 

 and plaster of the walls themselves. 



Exercise for Student. 



Devise experiments whereby it may be shown that the cooling 

 of one portion of a fluid sets up convection currents toward the 

 cooled region. 



