1875] WRITINGS OF JOSEPH HENRY. 491 



oil of the same quality. From different casks samples are 

 taken in tin canisters of a capacity of about half a gallon, 

 each canister being marked with the number of the lot and 

 the cask from which the oil was taken. Before the sample 

 is drawn from the cask the oil within is thoroughly mixed 

 by rolling the cask or by stirring. The object of this is to 

 obtain in the sample an average amount of solid matter 

 which may be contained in the oil. 



The purest lard oil is that which is manufactured by sub- 

 mitting the solid leaf lard to great pressure during the 

 coldest period of winter. Oil of this quality is used for burn- 

 ing in small mechanical lamps ; it gives a bright flame and 

 does not incrust the wick. The light-house lamps however — 

 being of a much larger size and evolving a much greater 

 amount of heat, can consume oil of a coarser character ; and 

 indeed it has been found that oil containing a certain amount 

 of solid matter, provided the latter is not too much in quan- 

 tity to be consumed by the lamp, gives a higher illuminat- 

 ing power. On this account — before this fact was generally 

 known in the trade, complaints were made of the Light- 

 House Board giving the preference to oil which in the market 

 would not be considered of the first quality. 



The quantity of oil is estimated by weight, allowing 7*6 

 pounds per gallon. It is weighed in gross and afterwards 

 emptied into large tanks in an under-ground vault. The 

 empty barrels are next weighed ; the weight of these deducted 

 gives the net weight of the oil. 



Previous to the establishment of the general light-house 

 depot at Staten Island, from which all the supplies are now 

 distributed, and where the lamps and other light-house appli- 

 ances are prepared for immediate use, the oil was received at 

 various ports along the coast in accordance with terms of the 

 contract, and was stored until wanted for use, in cellars hired 

 for that purpose. 



After the introduction of lard oil however, the board con- 

 structed a spacious under-ground receptacle capable of con- 

 taining 50,000 gallons of oil, and retaining it during the 

 whole year at a temperature not to exceed G5° Fahrenheit. 



The under-ground vault contains five tanks, each of the 



