488 WRITINGS OF JOSEPH HENRY. [1875 



The standard adopted with which to compare all other 

 lights is (as we have said) that of the London sperm candle, 

 which (under ordinary conditions) burns 120 grains of sperm 

 per hour. If it burns more or less than this amount during 

 tiie trial, a correction of a proportional amount is made in 

 the results. 



This standard however is too small for determining the 

 power of large lamps, and for this purpose an intermediate 

 standard is provisionally adopted. For example — in deter- 

 mining the power of a lamp of the first order, the power of 

 a lamp of the fourth order is first obtained, and this is used 

 as a comparison with the larger lamp. 



In the case of the arrangement — at the Staten Island depot, 

 for photometrical measurements, three scales are employed, 

 diverging from a centre at which the lamp to be measured 

 is temporarily placed ; at the farther end of each scale is 

 placed a sperm candle to serve as the standard of compari- 

 son. These scales are of different lengths, one being 100 

 inches in length, another 150 inches, and the third 200 

 inches; besides these, one of the scales is occasionally re- 

 placed by one of 700 inches in length, which is put up in 

 sections. 



As the semi-diameter of the burner of the lamp and that 

 of the candle must be included in the length of the scale, a 

 portion of the latter at each end is cut off. In adjusting the 

 scales therefore to their places, the measurement must be 

 taken from the middle of each scale ; thus in the case of the 

 one of 200 inches in length, the middle of it must be just 

 100 inches from the centre of the lamp on one side and 100 

 inches from the centre of the candle on the other. In mak- 

 ing the examination, three observers (one at each scale) 

 simultaneously take the photometric readings, and the mean 

 of the three results is adopted as the candle-power of the light 

 under examination. 



In the examination of oil previous to purchase, (as before 

 stated,) a lamp of the fifth order is charged with the oil 

 in question, and when in a state of equilibrium of combus- 

 tion — is subjected to the trial. For greater precision ten read- 



