Chemical Apparatus. 1 1 9 



often a remedy, or opening a pane of glass. In 

 new houses, where the chimneys are generally 

 cold, the air is often condensed before it reaches 

 the top, and consequently falls back again. Old 

 fashioned houses often have their vents too wide, 

 and was it not for the openings in the room 

 there would not be a circulation. The second 

 defect is gusts of wind being now and then driven 

 down, though in general the smoke may rise well 

 enough. This does not depend on the con- 

 struction of the room or the chimney ; it arises 

 from another cause. As the air passes over the 

 house, it must, especially in streets, be necessarily 

 thrown into eddies and irregular waves, as water 

 in a river, from the stones, &c. Hence the wind 

 often strikes down the chimney, which then must 

 be raised above the level of the buildings that 

 occasioned the eddy; or a machine may be 

 placed at top to turn with the wind. 



To return from this digression. The heat in 

 furnaces is regulated by diminishing the aperture 

 above, or that below. Tiiis method of managing 

 a fire is called damping, and is best done by 

 varying the aperture below. In fig. 17. is re- 

 presented the common small furnace for melt- 

 ing ; A is the ash-pit where the air enters, C is 

 the fire-place, containing a covered crucible- 

 stand on a support of baked earth which rests on 

 the grate, D is the passage into E, the chimney. 

 At D is a cupel placed in the current of the 

 flame, and at F is an earthen or stone cover, to 



