172 Professor Thomas Thomson m Coal Gas. 



short sixes. An opaque body is placed on a sheet of paper horizon- 

 tally between the two flames, and it is so placed that the two shadows 

 formed by it are of equal intensity. The distance between this 

 opaque body and the flames is measured, and the light emitted by 

 each is as the square of that distance. Thus, if the distance between 

 the gas flame and the opaque body be two feet, while its distance from 

 the flame of the candle is only one foot, then the light given out by 

 the gas is four times as great as that of the candle. 



The light given by the combustion of a jet of Glasgow gas issuing 

 from an orifice of one-thirtieth of an inch in diameter and four inches 

 in height was as follows : — 



1. On the north side of the river := 2*68 candles. 



2. On the south side of the river := 1*77 do. 



This method of measuring the quantity of light appears at first 

 sight very simple ; but I found, on trial, that it was attended with so 

 many sources of error, that I was afraid to depend upon it. Fortun- 

 ately there is another method of much easier execution, which I found 

 much more satisfactory. 



The quantity of light given out during the combustion of coal gas 

 is very nearly proportional to its specific gravity. The heavier a gas 

 is, the slower does it issue from an orifice of a given diameter when 

 propelled by a given force. I measured the time which a cubic foot 

 of each gas took to issue from an orifice of one-thirtieth of an inch, 

 when propelled by a force such as to form a jet of flame, when lighted, 

 of four inches in length, and I considered the goodness of the gas as 

 proportional to this time. The result was as follows : — 



Certainly, in a commercial point of view, the value of the gas (the 

 price per cubic foot being the same in all) is exactly proportional to 

 the time that it takes to burn ; because the consumption in a given 

 time depends upon that time. 



If, therefore, a thousand cubic feet of gas be charged 8s. on both 

 sides of the river, it is clear that the consumers on the south side pay 

 at the rate of 9s. 4d. per thousand cubic feet, because they consume 

 7000 cubic feet in the same time that those on the north side con- 

 sume 6000. 



If Glasgow gas, Skaterig gas, and Lesmahagow gas are each charged 

 at 8s. per thousand cubic feet, the price paid by the consumers will be. 



