Of the Light manifested in Combustion. 213 



the middle of the field of the instrument, when the 

 apparatus is prepared for making an experiment. 



The other divisions of this scale of light are de- 

 termined in such a manner that the numbers which 

 they bear, which I call degrees, are everywhere as the 

 sqtiares of their distances from the middle of the field 

 of the photometer, where the two shadows are in con- 

 tact whose densities are to be compared and equalized. 



To fill the important station of a standard light, with 

 which all others are compared, I have chosen a wax 

 candle of the first quality, just eight tenths of an Eng- 

 lish inch in diameter, and which burning with a clear 

 and steady flame has been found to consume very reg- 

 ularly 108 grains Troy of wax per hour. 



To this standard light I have assigned the value of 

 100 degrees ; and it is always placed exactly opposite 

 to that division of the scale of the photometer which is 

 marked 100°. This division is, of course, at the dis- 

 tance of 31.62 inches from the middle of the vertical 

 field of the instrument, that marked 10° being at the 

 distance of 10 inches. 



In order to express in a commodious manner the 

 quantities of wax, tallow, oil, or other inflammable sub- 

 stance consumed in the experiments, I have supposed 

 the 108 grains Troy of wax consumed by the standard 

 light per hour to be divided into 100 equal parts 

 (= 1.08 grains) to serve as a standard weight in all 

 cases. The usefulness of this arrangement will be seen 

 hereafter. 



I have now to request the attention and the indul- 

 gence of the Royal Society while I use my best en- 

 deavours to give them a clear and distinct account of 

 my experiments and their results. 



