374 



• KNOWLEDGE • 



[Mabcii 3, 1882. 



there is noiK*. Tlio cat that sits on the hoarthrug lia« an 

 abundant .su|i)>ly of frcsli air, niul if we liud tracliual 

 lircathinj; aiicrturcs all down the sides of our tiodii's, as 

 oat<'rpillai-s have, those on our lower extremities niij?lit 

 onjoy the ventilation. If we squatted on th(! f^ound like 

 savages something miglit lie said of the (ire-hole ventilator. 

 But as we are addicted to sitting on chairs that rai.se our 

 hroothing apparatus eonsideralily aliove the level of the top 

 of the register, tlie maximum eHiciency of the How of cold 

 air in the lake holow is expressed hy tlie prevalence of 

 chilblains and rheiunutism.* 



The atmosphere in which our heads are imn\ersed is j)rac- 

 tically stagnant; the radiations from the lire, plus the animal 

 heat from our bodies, just warm it sufficiently to enable the 

 cool entering air to push it upwards aliove the chimney 

 outlet and the surface of ^tlie lower moving stratum, and to 

 keep it there in a condition of stagnation. 



If anybody doubts the correctness of this description, he 

 has only to sit in an ordinary English room where a good 

 tire is burning — the doors and windows closed, as usual — 

 and then to blow a cloud by means of pipe, cigar, or by 

 burning brown paper or othci-wise, when the movements 

 below and the stagnation above, which I liave described, 

 will be rend'Ted visible. If there is noborly nio\ing about 

 to stir the air, and the experiment is fairly made, the level 

 of the cool lake below will be distinctly shown by the 

 clearing away of the smoke up to the Ie\el of the top of 

 the regist<'r opening, towards which it may be seen to sweep. 

 .Vbove thi.s, the smoke-wreaths will remain merely waving 

 about, with slight movements due to the small ineiiualities 

 of temperatures caused by the fraction of heat radiated 

 into the room from the fi-ont of the fire. These move- 

 ments are chiefly de\eloped near the door and windows, 

 where the above-mentioned cascades are falling, and against 

 the walls and furniture where feeble convection currents 

 are rising, due to the radiant heat alisorbed by their sur- 

 faces. The stagnation is the most complete about the 

 middle of the room wliere there is the greatest bulk of 

 vacant air space. 



When the inlet under the door is of considerable dimen- 

 sions, there may be some escape of warmer upper air at 

 the top of the windows, if their fitting is correspondingly 

 defective. These, however, are mere accidents : they are 

 not a part of the \aunted chimney-hole ventilation, but 

 interferences with it. 



There is another experiment that illustrates the absence 

 of ventilation in such rooms where gas is burning. It is 

 that of suspending a canary in a cage near tlie roof. But 

 this is cruel ; it kills the l)ird. It would be a more 

 satisfactory experiment to substitute for the canary-bird 

 any wingless biped who, after reading the above, still 

 maintains that our fire-holes are effective ventilators. 



Not only are the tire-holes worthless and mischie\ous 

 ventilators themselves, but they render efficient venti- 

 lation by other means practically impossil)le. The 

 " Arnott's ventilator " that we sometimes see applied to 

 the upper part of chimneys is marred in its action by the 

 greedy " draught " below. 



The tall chimney-shaft with a (ire burning immediately 

 below it dominates all the atmospheric movement in the 

 house, unless anotlxT and more powerful upcast-shaft lie 

 somewhere else in comnuniication with the apartments. 

 But in this ca.se the original or ordinai-y chimney would 

 be converted into a downcast-sliaft pouring air downwards 

 into the room, instead of carrying it away upwards. I 



• Sinco tho above was writton, a correspondent in Paria tells me 

 that a cnricaturp exists, roprosenting a Frenchman enjoying an 

 open lire by stnndiiiirnn iua hc:nl in tho middle of the room. 



need not descriU- the sort of ventilation thus obtainable 

 whih' the tire is burning and smoking. 



Ktl'ective sanitary ventilation should supply gentle and 

 unifurmly-diU'used cuiTcnts of air of moderate and equal 

 temperature throughout the house. We talk a great deal 

 about the climate here and the climate there, and when 

 we grow old and can atlbrd it we move to liournemouth, 

 Torquay, Mentone, Nice, Algiers, ic, for lK;tter climates, 

 forgetting all the while that the climate in which we prac- 

 tically live is not that out-of-doors, but the indoor climate 

 of our dwellings, the which, in a properly-constructed 

 house, may be regulated to correspond to that of any 

 latitude we may choose. I maintain that the very first 

 step towards the best approximation to this which is 

 attainable in our existing houses, is to brick up, cement up, 

 or otherwise completely stop up, all our existing fire-holes 

 and abolish all our exi.sting fires. 



But what next 1 The reply to this will demand the 

 whole of another short essay. 



THE ELECTRIC TELEGRAPH. 



Bv W. Lyxd. 

 THE WIRES AND INSULATORS. 



THE manner in which a telegraph line is carried from 

 station to station must be familiar to all. The 

 conducting wires are passed through a bath of liquid zinc, 

 by which process they receive a coat of that metal. Zinc 

 being easily oxidised, is by the action of the atmosphere 

 converted into o.xide of zinc, and protects the iron from 

 the influence of moisture. The wires are suspended by 

 earthenware or glass supports called insulators, fixed upon 

 wooden posts, at intervals of about sixty yards. As 

 electricity has always a tendency to pass by the shortest 

 route possible to the earth, it will easily be understood 

 that if the wires were allowed to rest upon the wooden 

 poles, the current would make its escape before it reached 

 the receiving station. Wood, when seasoned with tar, is 

 a very poor insulator. Tliere must be some good non- 

 conducting substance between the wire and the post. There 

 are many forms of insulators. Glass ofl'ers the greatest 

 resistance to an electric current of any known material, 

 but electricians oliject to it on account of its hygroscopic 

 properties — a film of moisture collects upon glass in nearly 

 all states of the w-eatlier. On some lines ebonite is used, 

 but there are objections even to that excellent non- 

 conducting substance ; rain wets it easily, and its surface 

 soon becomes dirty and spong}-. Browni earthenware insu- 

 lators are the most common in this country : the glaze does 

 not craek, and although they have not so great a resistance 

 as glass, they are, on account of their cheapness and dura- 

 bility, used in preference to any other kind of non-con- 

 ducting supports. In spite of all precautions, however, 

 electricity finds a means of escape from the best insulated 

 lines. In wet weather the leakage is sometimes so 

 great that the signals on the recording instruments at 

 receiving stations are almost unintelligible. The steel in- 

 dicator of a single needle telegi-aph apparatus should strike 

 against two ivory pivots on the dial plate, in order that the 

 beats may be distinctly understood, but I ha\ e had to read off 

 messages in bad weather when the current was so weak 

 that the needle did not touch the pivots at all, and half 

 the letters had to be guessed. In rainy weather, coats of 

 moisture collect upon the wire, insulator, and post, and as 

 a natural result the electricity flows to earth. If there is 

 only a little loss at each post, the cuirent on a long line is 



