WARMING AND VENTILATING OCCUPIED BUILDINGS. 315 



special receiver or mixing-chamber, into which the cold air also enters 

 before passing into the distributing-pipes. 



43. General rules, theoretical and ])ractical. — Theory and experiment 

 agree in showing that, calling — 



A the sectional area of a chimney or air-shaft; 



H its height ; 



T' the temperature of the external air ; v, 



T the mean temperature in the shaft ; 



y the mean velocity of the air in the shaft ; 



K a numerical co-efficient, constant for each shaft, and depending 

 upon its size and position; and 



Q the volume of air passing through in a second, 

 we have the equations — 



Y = KV(T-T')H 

 and 



Q = KAV(T-T')H 



44. Consequences of these formulas. — It follows from these formulas 

 that the velocity, Y, of the air or smoke in a chimney is proportional — 



1. To the square root of the excess of the temperature of the gases 

 in the chimney over the temperature of the external air; 



2. To the square root of the height of the chimney, and that the 

 volume of air or smoke discharged in a second is proportional to the 

 same quantity and also to the sectional area of the flue. 



It follows then — 



1. That the velocity, V, and, consequently, the volume, Q, of the gases 

 given off by the chimney are increased, or the draught rendered 

 stronger by increasing the height of the chimney. 



2. That the volume of gases or air removed are increased by giving 

 a greater sectional area to the chimney. 



3. That having given the height, sectional area, and general arrange- 

 ment of a chimney, or any shaft whatever for air or gas, the volume of 

 air which it will remove will always be the same if the temperature 

 within the shaft always exceeds that of the external air by the same 

 number of degrees. 



The latter consequence, perfectly confirmed by observation, renders it 

 necessary to proportion the heating-apparatus, which produces the cur- 

 rent for the case, when the temperature of the external air is greatest, 

 and consequently for the summer-season. 



45. Difference of temperature usually sufficient. — Observations made in 

 mines where the circulation of air is the most extended and complicated, 

 as well as of ventilating arrangements of the largest hospitals and 

 amphitheaters, show that a difference of 36^ to 45° between the temper- 

 ature in chimneys and that of the external air is usually sufficient to 

 produce, throughout Jihe air-passages, the velocities which will be men- 

 tioned further on. 



