270 Dr Andrew Fyfe on the Comparative Value of 



measuring the height of flame, and with a pressure-gauge, 

 to which a graduated scale, with a vernier, was applied, and 

 by which the pressure at the burner, necessary to keep up 

 the combustion at the desired height of flame, could be 

 measured to lOOths of an inch. The diameter of the pressure- 

 gauge was half-an-inch, which, though it gave a considerable 

 curve in the fluid, yet afforded more accurate results than when 

 one with a smaller diameter was used. Indeed, I had re- 

 course to various contrivances, such as floats in the water, 

 with indices attached to them, and made to move on the 

 scale ; but I found, after numerous trials, that the simple 

 wide gauge was by far the best, provided that care was 

 taken to mark the height of the fluid from the same part of 

 the curve. I always marked it from the bottom ; and for this 

 purpose had brass plates passing round the tube, and move- 

 able, so that they could be brought to the curve in both limbs. 

 In the following set of experiments, the gases were pre- 

 pared from a variety of coals by means of the experi- 

 mental apparatus. They were always consumed with a 5-inch 

 flame. The illuminating power was tried by the chlorine 

 test. The specific gravity was taken in the usual way, by 

 filling a vessel of known capacity with the gas, and noting 

 at the same time the state of the thermometer and baro- 

 meter. Not less than two trials were made with each coal, 

 and the experiments with each gas were frequently repeated ; 

 and when it was necessary, other gases were made, and the 

 trials again repeated, so as to secure accuracy. The con- 

 sumpt of the gas was ascertained in the usual way, by an 

 accurate experimental metre ; the trials with the specific 

 gravity, the pressure, and chlorine, being frequently repeated. 

 Instead of giving the names of the coals, I designate them 

 by letters. 



