118 Chemistry. [Lecture 28. 



son of this difference is, that as the heat in sum- 

 mer, and cold in winter, do not penetrate deep 

 beneath the surface, the internal parts of the 

 earth are generally of an equal temperature be- 

 tween the heat of summer and the cold of win- 

 ter; so that in winter the air getting in at the 

 drift is warmed by the air in the bowels of the 

 earth, and -ascends by the shaft. Again, in 

 summer, the air in the bowels of the mountain 

 being colder than the external column of air in 

 the shaft, becomes condensed, and flows out at 

 the drift, its place being supplied by the air flow- 

 ing in at the shaft. But when the external and 

 internal air are nearly of the same temperature, 

 as it is sometimes in spring and autumn, the cir- 

 culation is greatly impeded ; a circumstance dan- 

 gerous to the workmen, and which they obviate 

 by making large fires in the mines. 



It may not be quite improper, perhaps, in this 

 place, to notice the ventilation of air in chimneys, 

 which are liable to two faults : first, the want 

 of a draught up the chimney ; the second, the 

 wind being driven down, and the smoke along 

 with it, into the room. The first must arise 

 from some obstruction, as too much soot, or 

 from the too great closeness of the room ; for in 

 modern rooms the finishings are so close, that 

 there is often not a succession of air to supply 

 the place of that which rises up the chimney, 

 therefore the smoke rises a little way, and re- 

 turns again into the room. Lessening the vent i 



