230 ANNUAL OF SCIENTIFIC DISCOVERY. 



is effected; fresh air forcing itself in, and having its temperature raised; 

 when, making its escape, it gives place to a further supply at a lower tem- 

 perature. 



The process just described might be greatly assisted by the admission, by 

 day and night, and at all seasons, of a certain quantity of cold ; air into the 

 principal passages of a house. An opening in a door or window, properly 

 protected from weather, and on the most sheltered side of the house, is all 

 that is required. Partition walls, and others which are battened, usually supply 

 a means of indirect communication, between floors and ceilings, with the roof 

 of the house. No better system of ventilation can be adopted than to admit 

 a properly regulated quantity of air, in this way, to particular rooms where it 

 is most needed. Taking the air from the top of the house, that is, from the 

 roof, instead of the bottom (the kitchen and other domestic offices), it is always 

 cool and wholesome. In many instances advantage might be taken of a 

 picture, or a looking-glass, or a book-case to conceal an opening in the 

 plastering, the nearer the ceiling the better, say ten or twelve inches square, 

 for the ingress of air. This should be covered with perforated zinc, to keep 

 out insects, and fitted up with a sliding-door adjustable at pleasure. 



In devising plans for ventilating, many persons are greatly troubled about 

 getting rid of the heated air, and they are disappointed because it will not 

 make its escape at any opening they may choose for it. It is easier to begin 

 by admitting a continuous supply of cool pure air ; just so much of it, accord- 

 ing to season, and temperature, and other circumstances, as shall be agreeable, 

 and yet its presence should not be indicated by creating a draught. If this be 

 well looked after, there need be no anxiety about what becomes of the vitiated 

 air. That must be displaced by the entrance of fresh air. Both kinds canuot 

 occupy the same place at the same time. This is ventilation on the truest prin- 

 ciples, and without risk of having the head almost blown off, or the feet frozen. 



The gas meter is now so generally used that any description of it is here 

 unnecessary. As an accurate and disinterested measurer, between buyer and 

 seller, it has no equal in commercial transactions. Its construction and mode 

 of operation, the working of the index, and its means of recording the quan- 

 tity of gas which passes through the machine, are easily explained and 

 quickly understood, by those who wish to possess the necessary information. 



In estimating the relative cost of gas light, as compared with the light from 

 tallow, wax, and oil, it has been already stated, that equal quantities of light 

 from each material form the basis of such calculations. 



The several standards of comparison and their respective prices are as fol- 

 lows, namely 



Tallow candles (dips) at Gd. per Ib. 



Do. do. (moulds) Sd. ditto. 



Composition candles 10c?. ditto. 



Wax ditto .^ 2s. M. ditto. 



Common lamp oil 5s. 6d per gal. 



Sporin ditto 12s. ditto. 



When the charge for gas is 65. per 1,000 cubic feet, a quantity sufficient to 

 produce light equal to that to be obtained from a pound of tallow (mould) 



