242 Di\ Bence Jones on Ventilation, [April 18, 



to eight times the square root of the number of feet in the fall. 

 Hence the velocity of the air in the chimney per second is eight 

 times the square root of the difference in the height of the two 

 volumes of air. From this, the friction of the air in the chimney 

 must be deducted ; this varies directly as the length, and as the 

 square of the velocity, and inversely as the diameter ; usually from 

 one-third to one-fourth, must be deducted, and then multiplying 

 by 60 the velocity of the air per minute is found ; and multiplying 

 the velocity per minute by the area of the chimney, the number of 

 cubic feet of air discharged per minute is known ; that is, when the 

 entrance of air is as free as the exit. 



Another method consists in determining the rate of motion of 

 the air per minute by an anemometer, by multiplying the area of 

 the narrower part of the chimney by the velocity, the number of 

 cubic feet of air per minute passing out of the room may be ob- 

 tained. 



The determination of the rate of motion of the air passing into 

 the room is still more difficult ; no two openings into the room 

 give the same velocity, even if they are the same size, unless the 

 temperature of the air on the sides of the two openings is exactly 

 the same. 



Moreover, in all the physical methods the temperature of the 

 external air is constantly changing ; and the heat of the air in the 

 chimney is liable to great variations ; and the occasional ventilation 

 caused by opening the doors and windows interferes with the 

 accurate determination of the amount of constant ventilation. 



Though the physical method alone is still very imperfect, yet, 

 with the physiological method, it constitutes almost all the evidence 

 that has hitherto been sought in doubtful cases. 



3. The chemical method consists in weighing or measuring the 

 products of combustion in -the room. These products are heat, 

 water, and carbonic acid ; possibly small quantites of other sub- 

 stances are produced, but they cannot be determined quantitatively. 

 Moreover, the animal heat is so easily lost, and other sources of 

 variations in the temperature so interfere with the measurement of 

 the heat produced in the body, that it can afford no help. 



The amount of moisture in the room and in the internal air may 

 be found by experiment ; and assuming that each adult man by 

 respiration produces 3857 grs. of water in 24 hours, the quantity of 

 moisture which would be present if the room were closed may be 

 determined by calculation ; hence the quantity of air which 

 has escaped may be known. The same method may be followed 

 with the carbonic acid, which is a poison, and though it exists in 

 the atmosphere still it is only in very small and nearly constant 

 amount. 



Hence the chemical method mainly consists in determining how 

 much carbonic acid exists in any space in a given time, when a 



