302 .LIGHTING BY GAS. 



many other troubles and inconveniences in the attempt to 

 produce gas on a small scale for any practical purposes, 

 but we might be willing to take the trouble once for the 

 sake of performing the experiment." 



" Oh yes," replied John ; " and I mean to try it if you will 

 help me. I mean to have a small pitcher and pour a little 

 water on every few minutes." 



Before leaving this subject of the management of gas, I 

 will add that there is an artificial mode of packing this 

 bulky commodity after it is evolved, by compressing it, 

 with great force, in metallic reservoirs made prodigiously 

 strong to resist the pressure. The gas is driven into these 

 reservoirs by means of forcing-pumps working with great 

 power. The French have adopted this system in Paris to 

 a considerable extent. The engraving represents a wagon 

 loaded with gas thus compressed. 



The interior of the wagon is occupied by nine cylinders, 

 which are made of copper, and are of enormous strength. 

 There is forced into each cylinder ten or twelve times as 

 much gas as it would naturally contain if the gas were of 

 its ordinary density; and as the expansive pressure of the 

 gas is in proportion to the quantity of it that is forced into 

 a given space, the whole interior surface of each cylinder 

 has a force pressing upon it from within outward, and so 

 tending to burst it, often or twelve atmospheres! 



For you must understand that pressure in mechanics is 

 measured by atmospheres. The pressure of the atmosphere 

 is reckoned at fifteen pounds to the square inch. The 

 actual pressure of the atmosphere varies from day to day 

 in the same place according to the quantity of air that 

 there may happen to be for the time being over the place, 

 and in different places according to their elevation above 

 the level of the sea ; but fifteen pounds to the square inch 

 is taken as the standard of measurement, or, in other words, 



