Scientific Lectures. 211 



equally illuminated on its two sides. If the light from the can- 

 dle which passes through the transparent ring is exactly equal to the 

 light from the gas which falls upon the surface adjacent, the ring can- 

 not be seen ; and we know that at that point the illumination is equal, 

 and that therefore the light is in proportion to the square of the distance 

 of the disk from the respective flames. If, for instance, it is four 

 times as far from the gas as from the candle, we know that the gas is 

 giving sixteen times as much light as the candle. This bar is care- 

 fully graduated, so that, without making the measurements of the 

 distances or the calculations, we can read from the scale the ratio of 

 the two flames. 



You may like to know the quality of the gas which we have here 

 in New York. It is not popular, I know, to say anything in favor 

 of the gas companies ; but I have tested the gas of all the companies 

 except the Harlem ; and I have found it to range from sixteen to 

 eighteen candles ; that is, five feet of gas per hour, burned in an 

 Argand burner, produces as much light as sixteen to eighteen sperm 

 candles burning at the rate of two grains per minute. I am con- 

 vinced from repeated experiments, that the three companies whose 

 gas I have tested, the ~New York, the Manhattan, and the Metropoli- 

 tan, are giving us gas which is of excellent quality. 



You ask, then, why is the light so bad ? The difficulty is not in 

 the quality of the gas. It is in the quantity. In the district in 

 which I live, we receive the gas from the Metropolitan Company. 

 It is excellent in quality ; but the mains are in many streets only 

 three inches in diameter, and we cannot get enough gas. There 

 is another difficulty; the pipes in our houses are put in by con- 

 tract, and are too small. Then we put a five-light metre into the 

 cellar, and attempt to burn fifteen or twenty burners. 



But the great difficulty, after all, is in the burners. Only a week 

 ago I perused a report of a committee in London, who had been 

 examining the subject of gas-burners, and they found that some kinds 

 of burners did not give half the light they should, although they con- 

 sumed as much gas as good burners. They found that the burners 

 did not average more than three-fourths the light the^as was capable 

 of giving. Estimating the cost of gas at $10,000,000 per annum, 

 no less than $2,500,000 were wasted from the use of poor gas burners. 



You can easily see that if the burner brings too much oxygen in 

 contact with the gas, it destroys the little particles of carbon. We 

 must guard them carefully in burning gas for illumination to produce 

 the best light. 



