492 INVESTIGATIONS RELATIVE TO ILLUMINATING MATERIALS 



rection is made by simple proportion, which is applied to the measure- 

 ment previously obtained. 



The lamps containing- the oil for trial are lighted and trimmed in an 

 adjoinmg- apartment. They are introduced into the dark room through 

 a window closed with a sliding shutter. In order to prevent an over- 

 flow of oil at the burner by the oscillation of the liquid in the reservoir 

 by the agitation of transfer, each lamp is placed on a small carriage 

 moving- on a railway, which, passing through the window, enables the 

 lamp to be placed in its position with rapidity, and without the slight- 

 est disturbance of the equilibrium of the oil. 



The temperature of the room is also noted, and, as far as possible, it 

 is kept at a heat of not far from 70°. For this purpose, during- warm 

 weather the inspection may be made at night. 



For reading the divisions on the scales in the dark room, a mirror is 

 employed to throw the light of the lamp under inspection on the grad- 

 uation. 



To exclude all extraneous light, the three candles and the lamp to be 

 tested are each surrounded by a cylindrical sheet-iron screen, painted 

 black, through which a hole a little larger than the flame allows the light 

 to pass along the scale 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 measurement. Tf it 

 gives a power less than 8 caudles, the oil is rejected. If it passes that 

 test, it is then allowed to burn undisturbed without being trimmed for 

 8 or hours longer, aud if it is fouud at the end of that time to exhibit 

 no diminution in the brilliancy of the light it is considered worthy ot 

 adoption, especially if after this it continues to burn 4 or 5 hours with 

 no perceptible diminution which can be detected with the naked eye. 

 Tlie best lard-oil will burn 16 hours without trimming. 



Each candle before the measurement commences is suft'ered to burn 

 until it has assumed a perfect and uniform rate of consu miction 5 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 absorb- 

 ing it by one end of a strand of candle-wick cautiously introduced. If 

 any portion of the spermaceti is suffered to run down the side of the 

 candle and drop off below, the correction for variation in burning- will 

 be worthless. 



All materials for the use of the Light-House Establishment are pur- 

 chased by contract in accordance with published specifications as re- 

 gards quality aud certain conditions. The award is given to the lowest 

 bidder, provided he can offer trustworthy surety as to his ability to 

 fulfill the contract. Preference is however given Avhen bids are equal, 

 or nearly so, 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 



