Nov. 21, 1884.] 



• KNO\VLEDGE ♦ 



423 



The three chief agents to which we wish to call attention 

 are : — I. The wind. II. The luw of " cUtruaion of gases.' 

 III. The movements of the atmosphere caused by columns 

 of air of unequal weights. 



I. The Wind. — Perhaps the agent which may, 2^'^''' 

 excellence, be said to regulate all our endeavours to venti- 

 late our houses, public buildings, house-drains, et hoc genus 

 omne, is that very uncertain customer, the wind ; and since 

 the movements in the atmosphere which are due to the 

 force of the wind cannot be perceived directly, it will be 

 best to fall back on some simile in order to establish a basis 

 on which to found our subsequent remarks. 



Doubtless, each of our readers has, at one time or 

 another, been standing on the platform of a station when 

 £in express train has rushed by, and will probably have 

 noticed how every light particle in the vicinity was sucked 

 in towards the path of the rapidly-moving train. Now, the 

 raison d'etre of this auction is simply this : Before the 

 approach of the train, a given body of air was occupying a 

 certain place, the train rushes on, and (pushing before it 

 that body of air) occupies (temporarily) its place. After 

 the passage of the train more air rushes in, dragging with 

 it all light particles near, to fill the place vacated, else there 

 would be a vacuum or, in other words, a space containing 

 nothing. 



The wind acts very much in the same way. A moving 

 body of air sets in motion all the air in its immediate 

 vicinity ; it drives air before it, and at the same time causes 

 a partial vacuum on either side of its path, towards which 

 all air in the vicinity flows at (or nearly at) right angles. 

 Here is the advantage to be reaped from the wind as a 

 ventilating agent, but, as we shall see presently, this 

 uncertain ally has also evil propensities, which must be 

 guarded against. 



The wind, then, blowing over the roofs of our houses, 

 causes a current up our chimneys and ventilating shafts at 

 right angles to the direction in which it blows. Thus we are 

 able to form a very useful alliance with this natural force, if 

 we, at the same time, can only manage to keep the upper hand! 



Sometimes the wind, however, may impede ventilation 

 by obstructing the exit of the " induced current " from any 

 particular opening, or by blowing down any chimney or 

 ventilating shaft. This is, indeed, one reason of our failing 

 so often in obtaining an efficient system of ventilation — all 

 may go well in a still atmosphere, but the pressure of the 

 wind has not been taken into account.* We are, in a measure, 

 indebted to Mr. Sampson Low, B.A., F.E. Met. Soc, for 

 showing the plan to be adopted in order to make the utmost 

 use of the wind as a ventilating agent, and at the same time 

 guarding against its ill eflects. Mr. Low has constructed 

 a "ventilator" or ventilating head, which, while affording 

 the wind a free passage over the orifice from which it is 

 intended to extract the air, prevents any downward wind- 

 pressure on that orifice. The accompanying diagram of 

 this instrument may enable our readers to more readily un- 

 derstand the following brief description. The ventilator 

 conaists of three essential parts — the "shaft," to which is 

 fixed the dome-shaped " wind-chest," and the " cap," which 

 surmounts the whole structure. 



Now the "dome" and the cap are fixed on to one 

 another in such relative positions that at whatever angle 

 the wind strikes the dome, its upward curves direct the 

 current straight across the " extracting orifice," situated in 

 the dome of the ventilator. 



Perhaps the most I'emarkable feature of this instrument 

 is the fact that if the wind strikes down vertically on the 



* This pressure has been known to vary from one ounce to one 

 pound, as the rate of the wind varies from three and a half miles 

 to fourteen miles per hour. 



cap, an exhaust current within the shaft is in the same way 

 set up. Though we say that this is " remarkable," yet we 

 think our readers will see at once the simplicity of the 

 principle involved, if they will but bear in mind our simile 

 of the " railway train" 



The apparatus may be examined at Messrs. Sharp &. Co.'s 

 establishment, 11, Holborn-circus, London. 



Passing from our discussion of the wind and Mr. Low's 

 ventilator, we come to other factors in this scheme of venti- 

 lation, without due attention to which, we venture to say, 

 not even Mr. Low's patent nor the vendors' exploitation 

 thereof will be found a very great success. 



Concerning the diffusion of gases, it may be stated that 

 every gas diffuses (that is to say, tends to become inti- 

 mately mixed with another) at a certain specified rate — 

 viz., at a rate "inversely proportional to the square root of 

 its density," or, to translate that somewhat high-sounding 

 phrase into plain English, we may state this stupendous 

 fact thus : " Four volumes of the gas hydrogen will diffuse 

 through a porous partition in the same time that it takes 

 one volume of the gas oxygen to do so, oxygen being 

 sixteen times heavier than hydrogen," 



From this it will 

 be seen that there is 

 a constant escape of 

 any foreign gas into 

 the surrounding at- 

 mosphere. "This dif- 

 fusing tendency is so 

 great as to obtain 

 even through brick 

 and sione in every 

 room that is not air- 

 Section, tight" (Pettenkofer). 

 The amount of purification thus ob- 

 tained is, however, for all practical 

 purposes nil; and, moreover, organic 

 substances are not affected by it. It 

 is, therefore, only to be regarded as 

 a subordinate factor in ventilation. 



With reference to movements pro- 

 duced bj' columns of air of unequal 

 weights, it may be said that we have 

 a very important agent to assist us if 

 we will (and to binder us if we 

 " won't " !), in' devising an efficient 

 scheme of ventilation. If the air in 

 a room is heated by the presf-nce of a 

 fire or animal life, or if it be impreg- 

 nated with moisture, such air will expand ; and, if there be 

 any outlet, a portion of it will escape, and tint which re- 

 mains behind will be lighter than an equal volume of the air 

 external to the compartment. This external air will then 

 rush in through every orifice until "equilibrium" is re- 

 established. But, as the fresh air which comes in gets heated, 

 it likewise expands and escapes, and there is, therefore, a 

 constant stream of air coursing through the compartment. 



From the above considerations, we think it will be appa- 

 rent to our readers (we are, be it understood, treating 

 merely of cases where the external air is colder than the 

 air in the room) that there are, so to speak, two chief forces 

 concerned in the removal of vitiated air from our rooms or 

 buildings. They are the vis a fronte, or the suction-power 

 which the wind exerts by passing over our chimneys or 

 ventilating-shafts, and the ris a tergo, ox. the expansion and 

 consequent ascension of the gases within the room, aided, 

 to some extent, by that property of " diffusion " on whict 

 we have touched. There is one other agent which we have 

 not as yet alluded to, but it is one to which Messrs. Sharp 



Elevation. 



