210 Transactions of the American Institute. 



Here we have an apparatus designed to determine the illuminating 

 power of gas. It is necessary that it should be used in a dark room, 

 in which all the walls are painted black, so that there shall be no 

 reflection. We call this apparatus a photometer, or light measurer. 

 We compare the gas flame with a spermaceti candle, burning at the 

 rate of two grains per minute. If, on weighing the candle at the 

 close of the experiment, we find that the consumption has been more 

 or less than two grains per minute, we must correct the results in 

 proportion. The gas-burner is intended to burn five feet per hour. 

 The meter is so arranged that we can determine the amount with 

 accuracy ; and we regulate the burner to consume, as nearly as pos- 

 sible, five feet ; and, at the conclusion of the experiment, make the 

 proper correction if the quantity consumed has varied from five feet 

 per hour. 



It is necessary that the pressure should be advantageous. Too 

 much pressure diminishes the light. There is frequently complaint 

 of too little pressure. The truth is, the lower the pressure at 

 which we can burn the gas, the more light we get from it. It is 

 impossible that the pressure should be uniform in a large city. 

 Near the gas-works, the burners may have a pressure of three 

 or four inches ; while at a distance there may be hardly enough 

 pressure to carry the gas to the burner. At one of the gas- 

 works in this city, it is necessary to have a pressure of live 

 inches, in order to produce a pressure at the other extremity of 

 the line of seven-tenths of an inch. Between these two points the 

 pressure gradually varies from one extreme to the other. It is 

 utterly impossible to have a uniform pressure through the whole 

 city. The most economical pressure to employ is half an inch ; 

 and we have here a governor, a floating bell, so balanced with 

 weights, that whenever the pressure tends to exceed half an inch, it 

 will rise and partially close the opening through which the gas flows. 

 It thus maintains a uniform pressure of half an inch. 



In using the photometer, we use a clock that strikes minutes, tell- 

 ing us when to observe the meter. In making the comparison, we 

 determine the relative amount of light upon the simple principle that 

 light emanating from a point, and falling upon a surface, varies in 

 intensity inversely with the square of the distance. Here is a disc 

 of paper, having a ring rendered transparent by paraffine. This is 

 placed between the two flames, that of the standard candle, at one 

 end of this bar, and that of the gas at the other end. Moving this 

 little disc back and forth upon the rod, we find the point where it is 



