GAS-METER 



607 



the variation of the water levels-within such narrow limits, that the measurement 

 may be considered perfectly just on all occasions. 



The hydraulic, or wet gas-meter in ordinary use, when properly constructed, 

 measures with great accuracy, and requires only a very slight pressure of gas to 

 work it; but it readily admits of fraudulent means being employed by the consumer, 

 so as to cause the instrument to under-estimate the amount of gas consumed ; whilst, 

 on the other hand, the condensation of moisture within the meter may so far elevate 

 the water-level as to cause an over-estimation of the consumption. The water in 

 the hydraulic meter is also liable to freeze in winter, thus completely stopping the 

 supply of gas ; at other times, especially when the meter is fixed in a comparatively 

 warm place, the gas, becoming saturated with aqueous vapour, subsequently deposits 

 water during its passage through the cooler portions of the pipes of distribution. 

 The water thus deposited collects in the lower portions of the pipes, breaking the flow 

 of gas into a succession of bubbles, and causing that flickering or dancing of the gas 

 flames which is so frequently a source of annoyance in winter. 



These defects and inconveniences are all obviated by the dry gas-meter, several 

 forms of which have of late years been invented. The first in point of date is Clegg's 

 patent dry gas-meter, an instrument displaying great ingenuity, although it has never 

 come into very extensive use. Figs. 1066 and 1067 show the construction of this 

 meter : the letters of reference are the same in both. 



1066 



10G7 



BB, fig. 1067, Represents a cylindrical vessel, about 3| inches in diameter and 

 4 inches deep, being the dimensions of a meter capable of measuring gas for three 

 burners, called a three-light meter. In this vessel are two glass cylinders FF, con- 

 nected together by the bent tube d. The cylinders being perfectly exhausted of 

 air, and half filled with alcohol, are made to vibrate on centres c, e, and are balanced 

 by the weight /. This instrument accurately indicates the excess of heat to which 

 either cylinder may be exposed upon the principle of Leslie's differential thermo- 

 meter. 



c is a hollow brass box, called the heater, about 4 inches long and an inch broad, 

 projecting out of the meter about 1 inch. At a issues a small jet of gas, which, when 

 inflamed, gives motion to the cylinders. 



The gas enters the meter by "the pipe A, and circulates throughout the double case 

 B. Having passed round the case B, a portion of it enters the top of the box c, by 



