INVESTIGATIONS RELATIVE TO ILLUMINATING MATERIALS. 491 



The staudard adopted with which to compare all other lights is, as 

 we have said, that of the London sperm caudle, which, under or<liuary 

 conditions, burns 120 grains of sperm per hour. If it burns more or 

 less than this amount during the trial, a correction of a i^roportional 

 amount is made in the results. 



This standard however is too small for determining the i^ower of 

 large lamjjs, and for this purpose an intermediate standard is provision- 

 ally adopted. For examjile, in determining 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 comi^arison with the larger lamp. 



In the case of the arrangement at the Staten Island depot, for photo- 

 metrical measurements, three scales are employed, diverging from a 

 center 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 stand- 

 ard of comparison. These scales are of different lengths, one being 

 100 inches in length, another 150 inches, and the third 200 inches; be- 

 sides these, one of the scales is occasionally replaced 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 can- 

 dle must be included in the length of the scale, a jjortiou 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 center of the lam]) on one side, and 100 inches from 

 the center of the candle on the other. In making the examination, three 

 observers simultaneously, one at each scale, take the photometric read- 

 ings, 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 we have said be- 

 fore, a lamp of the fifth order is charged with the oil iu question, and 

 when in a state of equilibrium of combustion it is subjected to the trial. 

 For greater precision ten readings are taken on one side of the scale, 

 and then the photometer is reversed and as many taken from the oppo- 

 site side. In this way the mean of sixty readings, twenty on each 

 scale, furnishes the data on which the character of the oil principally 

 rests. As a means of simultaneously weighing the candles for checking 

 the effects of their irregular burning, three balances are provided, each 

 of which bears one of the candles in a socket sui)j)orted by a metallic 

 link, through which the scale-beam i)asses and is attached to the hook 

 of the scale-pan below. 



On the opposite scale-pan a series of grain weights are placed, which 

 can be taken off without disturbing the equilibrium of the scale, by a 

 pair of pincers ; the interval of time during which a given grain weight 

 Is burned is marked by a watch. If the interval is equal to two grains 

 for each 'minute, the candle is burning at its normal ratej if not, a cor- 



