GAS, COAL 



internal distribution, when of small size, arc made of lead, copper, wrought iron, 

 or tin. 



Instead of a stopcock for Ictting-off the gas in regulated quantities from the gas- 



1055 1056 



holder, a peculiarly-formed water or mercurial valve- is usually employed. Fig. 1056 

 shows the mode of construction for a water trap or lute, and is, in fact, merely a gas- 

 holder in miniature. c DBF is a sqiiare cast-iron vessel, in the one side of which a 

 pipe A is placed in communication with the gasholder, and in the other, one with tho 

 main B. Tho movcable cover or lid H G IK has a partition, L M, in its middle. If 

 this cover be raised by its counterweight, the gas can pass without impediment from 

 A to B ; but if the counterweight be diminished so as to lot the partition plate L M 

 sink into the water, the communication of the two pipes is thereby interrupted. In 

 this case the water-level stands in the compartment A so much lower than outside of 

 it, and in the compartment B, as is equivalent to the pressure in the gasometer ; there- 

 fore the pipes A and B must project thus far above the water. In order to keep tho 

 water always at the same height, and to prevent it from flowing into the mouths of 

 these pipes, the rim c D of the outer vessel stands somewhat lower than the orifices 

 A B ; and hence the vessel may be kept always full of water. 



If a quicksilver valve be preferred, it may be constructed as shown in fig. 1057. 

 A B are the terminations of the two gas-pipes, which are made fast in the rectangular 



1057 



* ron Vess0 l Mt B i s an i 1 * 011 vessel of the 

 same form, which is filled with quick- 

 silver up to the level a, and which, by 

 means of the screw o, which presses 

 against its bottom, and works in the 

 fixed female screw c c, may be moved 

 up or down so that tho vessel M may be 

 immersed more or less into tho quick- 

 silver. The vessel M is furnished with 

 a vertical partition m ; tho passage of 

 the gas from A to B is therefore ob- 

 structed when this partition dips into 

 tho quicksilver, and from the gradual 

 depression of the vessel B by its screw, 

 tho interval between the quicksilver and 

 tho lower edge of tho partition, through 

 which tho gas must enter, may bo en- 

 larged at pleasure, whereby the pressure 

 of the gas in B may be regulated to any 

 degree. Tho transverse section of that 

 interval is equal to tho area of tho pipe, 

 or rather greater ; tho breadth of the 

 vessel M, from A to B, amounts to tho 

 double of that space, and its length to 

 tho mere diameter of A or B. Tho greatest height to which tho partition m can 

 rise out of the quicksilver is also equal to tho above diameter, and in this case 

 tho lino comes to the placo of b. The vertical movement of the outer vessel B is 

 secured by a rectangular rim or hoop which surrounds it, and is made fast to tho 

 upper part of tho vessel M, within which guide it moves up and down. Instead of tho 

 lever D D, an index with a graduated plato may bo employed to turn the screw, and to 

 indicate exactly the magnitude in the opening of the valve. 



