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POPULAR SCIENCE MONTHLY. 



permit the gas to blow through without being properly consumed. Be- 

 low the normal the flame decreases; above, the light is increased some- 

 what, but not by any means in proportion to the increase in the gas flow. 

 Thus we see that the satisfactory employment of gas as an illuminant 

 depends upon the maintenance of a pressure high enough to deliver 

 the required amount of gas, but not so high as to cause wasteful 

 consumption. 



Turning back now to the gas main, let us consider the pressures 



actually existing. Exhibit 1 is a photograph of a twenty-four-hour 

 record of pressure at a point not far from the works. The radial 

 lines represent time, and there is a line for each quarter of an hour. 

 The circles represent pressure, there being one circle for each tenth of 

 an inch. Starting at E, the point at which the record begins, and 

 following the irregular line clockwise, one may readily determine the 

 fluctuations of pressure and the time of their occurrence. Inter- 

 preting the diagram, we find that the pressure was slightly above 

 the normal until 4.30 p. m. (A), when the works began to raise 



