490 WRITINGS OF JOSEPH HENRY. [1875 



to the photometer. The trial-lamps are those of the fifth 

 order. Each (after it has been lighted) is allowed to burn 

 an hour before being submitted to the photometrical meas- 

 urement. If it gives a power less than 8 candles, the oil is 

 rejected. If it passes that test, it is allowed to burn un- 

 disturbed without being trimmed — for 8 or 9 hours longer, 

 and if it is found at the end of that time to exhibit no dimi- 

 nution in the brilliancy of the light, it is considered worth}'' 

 of adoption, especially if after this it continues to burn 4 or 

 5 hours additionally with no perceptible diminution that can 

 be detected with the naked eye. The best lard oil will burn 

 16 hours without trimming. 



Each candle before the measurement commences is suffered 

 to burn until it has assumed a perfect and uniform rate of 

 consumption: it should be prevented from guttering by 

 removing a portion of the melted spermaceti which may 

 accumulate in the cup at the top of the candle beyond the 

 power of the feeble incipient flame to consume, — by absorbing 

 it with one end of a strand of candle-wick cautiously intro- 

 duced. If any portion of the spermaceti is suffered to run 

 down the side of the candle and drop off below, the correc- 

 tion for variation in burning will be worthless. 



All materials for the use of the Light-House Establish- 

 ment are purchased by contract in accordance with pub- 

 lished specifications as regards quality and certain conditions. 

 The award is given to the lowest bidder, provided he can 

 offer trustworthy surety as to his ability to fulfill the con- 

 tract. When bids are equal, or nearly so, preference however 

 is given to the bidder who is a manufacturer of the oil, and 

 not a mere vender of the article. During the inspection* 

 permission is granted to the contractor to be present at the 

 operation, in order that he may be assured that full justice 

 is done him in the examination. After seeing the precision 

 with which the photometric and other processes are con- 

 ducted, he is generally fully satisfied as to the results ob- 

 tained, even though his oil may have been rejected. 



The oil is delivered in iron-bound casks, varying from 38 

 to 50 gallons. These are placed (previous to inspection) 

 under a shed and arranged in different lots, each containing 



