340 SCIENCE IN SHORT CHAPTERS. 



and then to blow a cloud by means of pipe, cigar, or by 

 burning brown paper or otherwise, when the movements 

 below and the stagnation above, which I have described, 

 will be rendered visible. If there is nobody moving about 

 to stir the air, and the experiment is fairly made, the level 

 of the cool lake below will be distinctly shown by the 

 clearing away of the smoke up to the level of the top of 

 the register opening, towards which it may be seen to 

 sweep. 



Above this the smoke-wreaths will remain merely wav- 

 ing about, with slight movements due to the small inequali- 

 ties of temperature caused by the fraction of heat radiated 

 into the room from the front of the fire. These movements 

 are chiefly developed near the door and windows, where 

 the above-mentioned cascades are falling, and against the 

 walls and furniture, where feeble convection currents are 

 rising, due to the radiant heat absorbed by their surfaces. 

 The stagnation is the most complete about the middle of 

 the room, where there is the greatest bulk of vacant air- 

 space. 



When the inlet under the door is of considerable dimen- 

 sions, there may be some escape of warmer upper air at the 

 top of the windows, if their fitting is correspondingly de- 

 fective. These, however, are mere accidents; they are not 

 a part of the vaunted chimney-hole ventilation, but inter- 

 ferences with it. 



There is another experiment that illustrates the absence 

 of ventilation in such rooms where gas is burning. It is 

 that of suspending a canary in a cage near the roof. But 

 this is cruel; it kills the bird. It would be a more satis- 

 factory experiment to substitute for the canary-bird any 

 wingless biped who, after reading the above, still maintains 

 that our fire-holes are effective ventilators. 



Not only are the fire-holes worthless and mischievous 

 ventilators themselves, but they render efficient ventilation 

 by any other means practically impossible. The "Arnott's 

 ventilator" that we sometimes see applied to the upper part 

 of chimneys is marred in its action "by the greedy " draught " 

 below. 



The tall chimney-shaft, with a fire burning immediately 



