274 



VENTILATION. 



plied to Rooms," lays down the following 

 rules ; and as they, to a certain extent, 

 apply also to forcing-houses, at least so 

 far as the velocity and circulation are 

 concerned, they may not be out of place 

 here. " When an opening is made in the 

 ceiling, or upper part of a room, the force 

 which produces motion in the air is the 

 same universal law which regulates the 

 falling of bodies, and is precisely simi- 

 lar to the motion of water in a siphon. 

 The total height from the floor to the 

 point of final escape of the heated air is 

 the height of the siphon. The force of 

 motion is the difference of weight between 

 this column of heated air and that of a 

 column of external air of the same height. 

 Now, air expands, when heated, 1 -480th of 

 its bulk for each degree of Fahrenheit ; 

 and the velocity of motion is equal to the 

 additional height which a given weight of 

 heated air must have in order to balance 

 the same weight of cold air. Thus, sup- 

 pose a room 12 feet in height, and the air 

 20° higher in the room than the external 

 temperature, the air will expand l-24th 

 of its bulk by the excess of temperature ; 

 therefore 12^ feet of heated air will ba- 

 lance 12 feet of air which is 20° lower in 

 temperature. The motion of fluids is 

 equal to the velocity which a solid body 

 would acquire by falling through a space 

 equal to the excess of height which the 

 lighter body must have in order to ba- 

 lance the heavier. This velocity is as the 

 square root of 16 feet is to 16 feet per se- 

 cond, so is the square root of the given 

 height to the velocity sought. This resolves 

 itself simply into multiplying the square 

 root of 16 feet by the square root of the 

 given height. But as the acquired velo- 

 city of a gravitating body is equal to twice 

 the space it falls through in a given 

 time, the number thus found must be 

 doubled. In the case of the room we 

 have before supposed — as the additional 

 height of the heated column of air is 6 

 inches, so the square root of 6 inches re- 

 duced to the decimal of a foot, multiplied 

 by the square root of 16 feet, and that 

 product multiplied by 2, will give 5.6 feet 

 per second as the velocity of the air. An 

 opening in the ceiling 1 foot square will 

 therefore discharge 336 cubic feet of air 

 per minute. 



" It will be perceived that here also, as 

 well as in the case of the circulation of 



water, if either the vertical height or the 

 excess of temperature be increased four- 

 fold, the velocity will, in either case, be 

 twice as rapid as before. But whatever 

 be the calculated velocity, the real dis- 

 charge will not be so great as this theore- 

 tical quantity, not only in consequence 

 of friction, but also because the air will 

 be cooled in its passage through the ven- 

 tilating tubes, particularly if they extend 

 beyond the roof of the building. This will 

 considerably lessen the discharge; and 

 we ought, therefore, to deduct a certain 

 amount from the calculation, which, on 

 an average, should be about one-fourth 

 of the whole quantity." 



" As the discharge through any given 

 height and size of ventilator is less in 

 proportion as the difference between the 

 external and internal temperature is 

 smaller, it follows that it will be most 

 difficult to obtain ventilation in hot 

 weather. In summer, either the number 

 or dimensions of the ventilator should 

 be increased," otherwise ventilation that 

 would be sufficient in winter would be 

 quite inadequate in summer. " The open- 

 ings for the admission of air," Hood ob- 

 serves, " should always be placed as near 

 to the floor as possible. In size they 

 should be larger than the area of the ven- 

 tilators," that is, the discharging ones, 

 "in order that the influx, of cold air may 

 not proceed with too great velocity and 

 cause a draught. In fact, the size of 

 these openings is a matter of indifference, 

 provided only that they be sufficiently 

 large ; for the quantity of air that enters 

 through them depends entirely on the 

 quantity that passes off through the 'top' 

 ventilators, and not vice versa; for if the 

 passages for the cold air were double the 

 size of the ventilators ' for its escape,' no 

 more cold air could enter than a quantity 

 equivalent to that of the heated air which 

 escapes at the 'top;' and none of the 

 heated air can escape at the cold-air chan- 

 nels, because heated air cannot descend. 



" The more numerous and divided are 

 the openings for the admission of cold air, 

 the less inconvenience will be experienced 

 by currents; but unless a sufficient quan- 

 tity of cold air be admitted in this man- 

 ner, there will be a counter current of 

 cold air forced through the ventilators, 

 which will descend and produce a very 

 disagreeable draught." 



