different kinds of Coal-gas Burners, ii^^c. 219^ 



consumpt was the same, or so very nearly so that the differ- 

 ence was unworthy of notice, in the experiments of the nature 

 of which I was engaged. 



I have already mentioned, that, according to Christison and 

 Turner, when the flame of a jet is gradually lengthened, the 

 light becomes in & greater and greater ratio than the consumpt 

 of gas, till it reaches five inches, beyond which there is no far- 

 ther gain ; and that this is the case, I have myself verified by 

 numerous trials. We may consider, then, five inches as the 

 most economical height for a single jet. Accordingly, in using 

 it as my standard, I have invariably so employed it, that I 

 might the more easily be enabled to compare the lights given 

 by other burners with it, and with each other, for equal con- 

 sumpts of gas. 



The method to which I resorted for judging of the illumi- 

 nating power is that of Rumford ; the depth of shadow af- 

 forded by the flames; of course, attending to the circum- 

 stances noticed by him which are necessary to secure ac- 

 curacy. 



I am aware that many object to this method, as being in 

 several respects fallacious; but these objections are of little 

 force, when the instrument is used for ascertaining the com- 

 parative illuminating power of lights, such as gas flames, where 

 the shadows do not vary much in their colour from each other. 

 The standard jet was generally kept at a fixed distance; the 

 other lights, after being brought to the proper height, were 

 moved till the shadows were the same ; and in doing this, I 

 was assisted by others in whose accuracy 1 could rely. I men- 

 tion this, to shew that I did not trust entirely to my own ob- 

 servation in judging of the shadows, and that reliance may the 

 more readily be placed in the results detailed. 



The gas used in the moveable burners was measured by a 

 gas-meter; and to secure still farther accuracy in the results, 

 a cubic foot passed through the metre was thrown into the 

 gasometer connected with the standard jet, and accurately 

 marked off on its scale. The pressure on the gas from the 

 metre varied from 15 to 19-lOths of an inch of water; that 

 on the standard gasometer was kept at 17. 



In the paper by Drs Christison and Turner, and also in 



