62 SCIENCE IN SHORT CHAPTERS. 



Below this level there is abundant ventilation, above it there 

 is none. The cat that sits on the hearth-rug has an abundant 

 supply of fresh air, and if we had tracheal breathing apertures 

 all down the sides of our bodies, as caterpillars have, those on 

 our lower extremities might enjoy the ventilation. If we 

 squatted on the ground like savages something might be said 

 for the fire-hole ventilator. But as we are addicted to sitting 

 on chairs that raise our breathing apparatus considerably above 

 the level of the top of the register, the maximum efficiency of 

 the flow of cold air in the lake below is expressed by the prev- 

 alence of chilblains and rheumatism.* 



The atmosphere in which our heads are immersed is practi- 

 cally stagnant ; the radiations from the fire, plus the animal 

 heat from our bodies, just warm it sufficiently to enable the 

 cool entering air to push it upward above the chimney outlet 

 and the surface of the lower moving stratum, and to keep it 

 there in a condition of stagnation. 



If anybody doubts the correctness of this description, he has 

 only to sit in an ordinary English room where a good fire is 

 burning the doors and windows closed, as usual and then to 

 blow a cloud by means of pipe, cigar, or by burning brown 

 paper or otherwise, when the movements below and the stag- 

 nation above, which I have described, will be rendered visible. 

 If there is nobody moving about to stir the air, and the experi- 

 ment 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, toward which it may 

 be seen to sweep. 



Above this the smoke-wreaths will remain merely waving 

 about, with slight movements due to the small inequalities 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-men- 

 tioned 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. 



AVhen the inlet under the door is of considerable dimen- 



* Since the above was written, a correspondent in Paris tells me 

 that a caricature exists, representing a Frenchman enjoying an open 

 nre by standing on his head in the middle of the room. 



