214 



SCIENCE OF COMMON THINGS. 



"Why smoke ascends the chimney. 



What is the draught of a chimney ? 



Because the air in a room is not subject to much 

 change, and soon partakes of the same temperature as 

 our bodies, when it no longer feels cold. 



1437* Why do we generally feel colder out-of-doors than in-doorsf 



Because the air (which surrounds us) is always 

 changing / and as fast as one portion of air has become 

 warmer by contact with our body, another colder por- 

 tion surrounds us, to absorb more heat. 



1458 Why is there always a draught through the window crevices ? 



Because the external air (being colder than the air 

 of the room we occupy) rushes through the window 

 crevices, to supply the deficiency caused by the escape 

 of warm air up the chimney, etc. 



1459 Why, when we kindle a fire in a stove or grate, does the smoke 

 ascend the chimney ? 



When a fire is lighted to warm a room, the smoke 

 and other gaseous products of combustion, being lighter 

 than the air of the room, ascend, and soon fill the chim- 

 ney with a column of air lighter, bulk for bulk, than 

 a column of atmospheric air. 



143O /s the column of light air in the chimney pressed up by a column 

 of equal size on the exterior of the chimney f 



It is. 



1431 What, then, is the draught of 

 a chimney f 



It is the rate or speed with 

 which the column of cold air 

 outside the chimney pushes 

 up the column of warm air 

 inside the chimney, and this 

 draught will be strong and 

 effective just in the same 

 proportion as the column of 

 air in the chimney is kept 

 warm. 



Fig. 47 represents a section of a 

 grate and chimney. C D represents 

 the light and warm column of air with- 

 in the chimney, and A B the cold and 

 heavy column of air outside the chim- 



A! 



47 



