$2 Count Rum ford's Account of a Method of 
depending in a great measure upon the length to which the 
wick is drawn out, yet I have found by repeated trials that 
this lamp, once properly adjusted, continues to emit light 
more equally for a considerable time than any other lamp, 
and much more so than any candle whatever. 
At the beginning of each experiment I adjust this standard 
light in the following manner. Having placed the lamp 
upon its carriage, at the distance of too inches from the centre 
of the field of the photometer, measuring from the centre of 
the circular flame of the lamp, a .cylindric wax-candle, ot 
known weight and dimensions, and which is kept merely for 
that purpose, being lighted and trimmed, and made to burn 
with the greatest possible degree of brilliancy, is placed over 
against it, at a certain given distance (33 inches), and then 
the wick of the lamp is drawn out, or shortened, as it is found 
necessary, till the shadows corresponding to the lamp and to 
the candle, are precisely of the same density ; this done, the 
proof candle is extinguished, and laid by for further use, and 
the projected experiment is immediately commenced. 
Here the proof candle is, properly speaking, the standard, 
but the lamp is to be preferred to it for the experiments, on ac- 
count of the superior constancy or equality of its light. 
The only danger of error in this matter arises from the dif- 
ficulty of procuring proof candles, which shall always give 
precisely the same quantity of light, or of making the same 
candle burn with exactly the same brilliancy at different times, 
I flattered myself at one time that even this cause of error and 
uncertainty, however insurmountable the difficulty appears, 
might be in a great measure removed. I conceived that if tne 
light from the standard lamp and that of the proof candle, 
