238 On Warming and Ventilating 



number may be taken as a constant quantity, expressive of the 

 power of any stove ; but it also expresses the weight of air in 

 pounds, which one pound of Newcastle coal heats one degree 

 of Fahrenheit's thermometer. 



This number will not be strictly a constant quantity, as small 

 stoves will not act quite to the same advantage as larger ones ; 

 and local and other circumstances will in some degree alter the 

 result of experiments made in the manner above stated. This 

 is more especially the case, when the admission of cold air 

 and the discharge of foul air, are in any degree influenced by 

 the wind. 



The cold air is generally brought directly from the atmo- 

 sphere ; and, therefore, as its progress along this channel is af- 

 fected by the wind, a greater or less quantity will pass through 

 the stove. If the air be deficient, less heat is carried off from 

 the heating surface, and a greater proportion goes up the chim- 

 ney ; on the contrary, when the wind blows into the cold air-flue, 

 the two forces conspire, more air is admitted, more heat is car- 

 ried off with the air, and of course less is wasted up the 

 smoke-flue. 



In all situations where it is practicable, I use an effectual 

 means of regulating the admission of cold and the escape of 

 foul air, by placing at the commencement and termination of 

 these apertures a turn-cap or cowl, in which the vanes are so 

 fixed as to let the wind blow into the one, and assist the escape of 

 air from the other. Although this contrivance will always pre- 

 vent a counter current, which without its use is sometimes the 

 case ; it does not prevent unequal quantities of air from enter- 

 ing, according to the strength of the wind. This is not found in 

 practice to be a great inconvenience ; for during the most per- 

 fect calm, the air admitted by the power of the stove alone, is 

 sufficient for every purpose of warmth and ventilation : whilst 

 with a tolerable fire in the stove when the wind is considerable, 

 the air comes into the rooms at a higher temperature than the 

 rooms require which is at least erring on the desirable side. If 

 the quantity of air admitted under all states of the wind were 

 required to be uniform, the aperture in the turn-cap for cold air 



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