612 TRANSACTIONS OF THE AMERICAN INSTITUTE. 



althor.gh the velocity of the column may be greatly increased, its 

 volume, also-, having been greatly increased by expansion, the 

 actual quantity of air entering at the bottom will be lessened. 

 The limit to which the ascending column may be heated, is about 

 500° F. Any additional heat beyond this point will diminish the 

 draft. Thus nature interposes a law to prevent too great a waste 

 of heat. 



6. A flue of large area or capacity is fiivorable for drafts, 

 because the amount of friction of the ascending column is pro- 

 portionately less. 



7. Too large a flue diminishes the draft, because the smoke, 

 moving at a slower rate, is longer exposed to a greater surface of 

 the chimney, and drags with it more cold air, drawn in through 

 other openings than that beneath the fire. 



8. A certain quantity of air is necessary for combustion; if it 

 should be entirely shut ofi" below the tire, and two flues were con- 

 nected therewith, the cold air would pass down one flue, to supply 

 the oxygen needed for combustion, and the smoke would pass up 

 the other flue. 



9. Too great a supply of air is detrimental. It dampens the 

 fire, cools the ascending column, checks the draft, and lessens 

 combustion. 



10. A chimney gradually increasing in capacity from the bottom 

 upward, tends to increase the draft. 



11. A chimney flue gradually increased in size from the top 

 downward, for a short distance, will lessen the eflTect of downward 

 currents produced by gusts of wind. 



12. Wind blowing horizontally generally increases the draft, 

 because any current moving in a right line tends to draw with it 

 another like current moving perpendicularly to it. 



13. Wind will retard the draft, provided there are open win- 

 dows in the room from which the draft is supplied, which are 

 parallel with the course of the wind; for, in that case, the so-called 

 suction-force of the wind will be greater below than above. 



14. The best draft and most economical fire is secured by closing, 

 air-tight, the lower portion of the chimney, and every part of the 

 heating apparatus except the damper, which regulates the fire and 

 which, to produce perfect combustion, should admit two-thirds of 

 the air used under the fire, and one-third above it. By this means 

 the combustible gases, which often pass ofl' unconsumed, coming 



