March 3, 1882.] 



KNOWLEDGE 



373 



platinum wire stretched across the cup. When the firing 

 circuit is completed, the current enters by means of a wire 

 joined to W, and, passing this, the platinum leaves by 

 another wire joined to W. Fifty to sixty Leclanche cells 

 are used, and make the platinum red-hot, thereby igniting 

 a small quantity of gun-cotton wrapped round it, the flame 

 from which, consuming a thin paper cover P P', enters the 



detonating tube T T, containing fulminating mercury. 

 The e.xplosion of this is followed by the explosion of the 

 mine. When the mine is fired by hand, a key, somewhat 

 similar to a Morse telegraph key, is used. Normally there 

 is a piece of ebonite switched into the space between the 

 contact points, so that the mine may not be accidentally 

 fired. The whole collection displays the state of perfec- 

 tion to which we have attained in the art of wholesale 

 homicide. 



" Field Telegraph Equipment," "Signalling Apparatus," 

 and "R.K Field Company Equipment," are all well repre- 

 sented, but our space will not permit a description this 

 week. We can only say that everything gives one an 

 impression of extreme compactness and efficiency, reflecting 

 great credit on the War Oflice. 



A new feature in the Exhibition is the delivery of a 

 course of lectures by Professor Sylvanus Thompson. In 

 the first he essayed to demonstrate what electricity is, and, 

 after describing its production and effects, he said that the 

 late Professor Clerk Maxwell regarded electricity as the 

 motion of the ether of space, and that it was most proliably 

 in that direction we should have to look to discover the 

 nature of electricity. 



VENTILATION BY OPEN FIRE- 

 PLACES. 



By W. Mattieu Willi.uis. 



THE most stubborn of all errors are those which have 

 been acquired l>y a sort of inheritance, which have 

 passed dogmatically from father to son, or, still worse, 

 from mother to daughter. Tliey may become superstitions 

 without having any theological character. The idea that 

 the weather changes with the moon, that wind " keeps ott' 

 the rain," are physical superstitions in all cases where they 

 are blindly accepted and promulgated without any exami- 

 nation of evidence. 



The idea that our open fireplaces are necessary for ven- 

 tilation is one of these physical superstitions, which is 

 producing an incalculable amount of physical mischief 

 throughout Britain. A little rational reflection on the 

 natural and necessary movements of our household atmo- 

 spheres demonstrates at once that this dogma is not only 

 baseless, but actually expresses the opposite of the truth. 

 I think I shall be able to show in what follows that, 

 1st, they do no useful ventilation ; and, 2nd, that they 



render systematic and really efleetive ventilation practically 



inipo-ssible. 



Everybody knows that when air is heated it expands 

 largely, becomes lighter, bulk for bulk, than other air of 

 lower temperature ; and, therefore, if two portions of air of 

 unequal temperatures and free to move are in contact with 

 each other, the colder will flow under the warmer, and 

 push it upwards. This latter postulate must be kept 

 distinctly in view, for the rising of warm air is too 

 commonly regarded as due to some direct uprising activity 

 or skyward affinity of its own, instead of being understood 

 as an indirect result of gravitation. It is the downfalling 

 of the cooler air that causes the uprising of the warmer. 



Now, let us see what, in actordante with the above- 

 stated simple laws, must happen in an ordinary English 

 apartment that is supplied, as usual, with one or more 

 windows more or less leaky, and one or more doors in like 

 condition, and a hole in the wall in which coal is burning 

 in an iron cage immediately beneath a shaft that rises to 

 the top of the house, the tire-hole itself having an extreme 

 height of only 24 to 30 inches above the floor, all the 

 chimney above this height being entirely closed. (I find 

 by measurement that 24 inches is the usual height of the 

 upper edge of the chimney opening of an ordinary " re- 

 gister" stove. Old farm-house tire-places are open to the 

 mantel-piece. ) 



Now, what happens when a heap of coal is burning in 

 this hole ? Some of the heat — from 10 to 20 per cent, 

 according to the construction of the grate — is radiated into 

 the room, the rest is conveyed by an ascending current of 

 air up the chimney. As this ascending current is rendered 

 visible by the smoke entangled with it, no further demon- 

 stration of its existence is needed. 



But how is it pushed up the chimney 1. Evidently by 

 cooler air, that flows into the room from somewhere, and 

 which cooler air must get under it in order to lift it. In 

 ordinary rooms this supply of air is entirely dependent 

 upon their defective construction — bad joinery : it enters 

 only by the crevices surrounding the ill-fitting windows 

 *nd doors, no specially-designed opening being made for it 

 Usually the chief inlet is the space under the door, through 

 which pours a ri\ ulet of cold air, that spreads out as a lake 

 upon the floor. This may easily I* proved by holding a 

 lighted taper in front of the bottom door-chink when the 

 windows and other door — if any — are closed, and the fire 

 is burning briskly. At the same time more cold air is 

 poured in at the "top and the side spaces of the door and 

 through the window chinks. The proportion of air entering 

 by these depends upon the capacity of the bottom door- 

 chink. If this is large enough it will do nearly all the 

 work, otherwise every other possible leakage, including the 

 key -hole, contributes. 



But what is the path of the air which enters by these 

 higher level openings 1 The answer to this is supplied at 

 once by the fact that such air being colder than that of the 

 room, it must fall immediately it enters. The rivulet 

 under the door is thus supplemented by cascades pouring 

 down from the top and sides of the door and the top and 

 sides of the windows, all being tributaries to the lake of 

 cold air covering the floor. The next question to be con- 

 sidered is, what is the depth of tliis lake 1 In this, as in 

 every other such accumulation of either air or water, the 

 level of the uppi-r surface of the lake is determined by that 

 of its outlet. The outlet in this case is the cliimney hole, 

 through which all the overflow pours upwards : and there- 

 fore, the surface of the flowing stratum of cold air corre- 

 sponds with upper part of the chimney hole, or of the 

 register, where register stoves are used. 



Below this level there is abundant ventilation, above it 



