CHAMBERS'S INFORMATION FOR THE PEOPLE. 



also required in stables, cow-houses, and other 

 places where valuable animals are kept. 



Ventilation consists of two operations the re- 

 moval of the foul air, and the introduction of fresh. 

 Though neither operation can go on without the 

 other going on at the same time, it is convenient 

 to consider the two separately. 



The agents employed in removing the air from 

 apartments are chiefly two : that by which nature 

 effects the ventilation of the earth on a grand scale 

 (see METEOROLOGY) namely, the draught of 

 ascending currents produced by difference of tem- 

 perature ; and mechanical force, such as pumping. 

 The former is the more common, and is the only 

 one applicable to private houses. 



The column of air in the chimney of a lighted 

 fireplace being expanded and comparatively light, 

 exerts less than the prevailing pressure on the air 

 immediately under and about its base. On the 

 principle, therefore, explained in HYDROSTATICS 

 and PNEUMATICS, the air below and around it 

 pushes it up, and flows in to take its place ; the 

 velocity of the movement being in proportion to 

 the height of the chimney and the degree of heat. 

 Thus, although it is often convenient to speak of 

 the air being drawn or sucked into the chimney, 

 the force does not lie in the chimney, but in the 

 greater pressure of the air behind. 



Wherever, then, there is a heated chimney, there 

 is a means of removing the foul air. And in rooms 

 moderately lofty and spacious, with windows and 

 other fittings not closer than usual, and a chimney- 

 mouth of the usual width, there is little risk, when 

 there are only a few inmates, of any serious vitia- 

 tion of the air. The ascent, however, of foul 

 heated air to the top of the room dictates its 

 exit in that direction, rather than low down at 

 the mouth of the chimney. It is conceived by 

 some that the carbonic acid of the breath, from 

 its greater weight, must be chiefly at the bottom 

 of the room ; but this is a mistake. The heated 

 breath ascends instantly, because it is, as a whole, 

 lighter than the air around it ; and the carbonic 

 acid in it does not tend to separate from it and 

 fall down by its superior weight, but, by the law of 

 the diffusion of gases (see METEOROLOGY), seeks 

 to spread itself equally all over the room, and 

 would do so though it were lying at first on the 

 floor. It is on the principle of the foul air ascend- 

 ing at first to the top of a room that Dr Arnott's 

 ventilating-valve is contrived. The valve may be 

 used to supplement the open-fire draught in small 

 and crowded apartments, and is essential where 

 the fire is burned in a close stove or in the smoke- 

 less grate. The valve is represented at z/, fig. I. 

 An aperture is cut in the wall over the chimney, 

 as near to the ceiling of the apartment as may be 

 convenient. In this is suspended a valve, capable 

 of opening inward to the chimney, but not in the 

 other direction, by which means a return of smoke 

 is prevented. It operates by virtue of the draught 

 in the chimney. A wire descends to a screw or 

 peg fixed in the wall, by which the opening of 

 the valve may be limited or altogether prevented. 

 This is a far more efficient plan of ventilation 

 than an open window, or an opening in the wall 

 near the roof, leading merely to the outer air; 

 where there is an open fire in the room, such 

 openings rather admit a rush of cold air than let 

 out the foul. 



There is generally more or less draught in a 



438 



chimney even without a fire, from the air within 

 being slightly warmer than that without ; and this 

 action might be strengthened by burning a jet of 

 gas within the ventilating aperture at v. Where a 

 house is to be built new, some recommend having 

 special ventilating-flues in the walls, separate from, 

 but close to the fire-flues, so that the air may be 

 heated, and an ascending current produced. In 

 weather when fires are not required, the draught 

 can be maintained by gas-jets at the entrances to 

 the vents. This plan of causing a draught by gas 

 is applicable to churches and apartments without 

 fireplaces. 



Where a fire is burned for the express purpose 

 of producing a current of air, it is called ventilation 

 by 'fire-draught. The plan has been exemplified 

 with success in mines, where a fire being lighted 

 at the bottom of a shaft, air is drawn off in all 

 directions around, and sent up the shaft ; to re- 

 place which, fresh air is constantly pouring down 

 other shafts. 



Many of our large buildings are ventilated by 

 fire-draught. Fig. 5 shews an arrangement by 

 which a school or 

 church may be venti- 

 lated : aa, the floor- 

 ing perforated with 

 holes, through which 

 air, warmed by hot- 

 water pipes, passes 

 to the interior. The 

 ceiling, 66, is per- 

 forated, leading to a" 

 chamber which com- 

 municates with a ver- 



Fig- 5- 



tical flue, cc; which leads to the fireplace of the 

 warming-apparatus, situated at the foot of a flue, ed. 

 As the only air which reaches this must pass from 

 cc, a constant current is maintained therein, and 

 also through the apertures in the ceiling. Dr Reid 

 exemplified this method, first in his own class-room 

 in Edinburgh, and afterwards in various public 

 buildings, among others, in the temporary House 

 of Commons, erected after the burning of the old 

 house in 1834. The arrangements for warming 

 and ventilating the present House of Commons 

 are a modification of Dr Reid's plan. 



In other cases, as at the prison in Millbank, 

 warm air is admitted at the ceiling, and carried 

 off by the draught of a chimney in connection 

 with the sides or lower part of the rooms. 



In these last-mentioned instances, the apparatus 

 provides as well for the admission as for the 

 removal of air. In ordinary dwellings, no special 

 provision is in general made as to admission. 

 It is, in fact, not absolutely necessary ; for the 

 removal of a portion of the air of a room never 

 fails to secure the entrance of a fresh supply some- 

 where. Whenever the chimney-draught or other 

 means removes a little of the pressure inside the 

 room, the pressure without forces air through every 

 opening and chink. But this irregular source of 

 supply has various inconveniences. It often re- 

 quires more force to strain the air in this manner 

 than the draught is possessed of, and then the 

 chimney smokes ; it is smoke produced by this 

 cause that is curable by opening the door or 

 window. For these and other reasons, there ought, 

 in all cases, to be a free and legitimate entrance 

 provided for fresh air, so as to give a control over 

 it ; and this entrance should be independent of 



