1875] WRITINGS OF JOSEPH HENRY. 497 



oil which it contains. The oil in the reservoir in this con- 

 dition is supported by the pressure of the atmosphere on tlie 

 surface of the oil in the cup. When this surface is lowered 

 b}^ burning, the end of the tube is opened, and a bubble of air 

 passes up and an equal bulk of oil descends, and in this 

 way a nearly constant level of oil is maintained. I say 

 nearly constant because the air which goes up is of some 

 volume and in the act of passing up produces an oscillation 

 which in some degree affects the steadiness of the flame. 



There is however a greater defect in this lamp from the 

 oscillations in the level when the reservoir has been ex- 

 hausted of a considerable portion of its charge of oil. In 

 this case the arrangement is one similar to an air thermom- 

 eter with a large bulb, and is affected by a sudden draught 

 produced by the opening and shutting of a door or the 

 ordinary ventilation of the lantern. This was partly rem- 

 edied by bending the tube, and thereby increasing the resist- 

 ance to a sudden change in the level of the oil. 



The improvement of Mr. Funck consisted in substituting 

 for these lamps one of constant level, in which the oil is 

 placed above the burner, and the flow of oil necessary for 

 perfect combustion is regulated by a small floating piston, 

 placed in an enlarged portion of the supply-tube, and carry- 

 ing on its upper surface a conical projection which increases 

 or diminishes the size of the supplying orifice in accordance 

 with the rapidity of combustion. This lamp is not onl}'- free 

 from the objections pertaining to the other' lamps, but is less 

 expensive and better adapted to the burning of lard oil. It 

 affords a freer combustion, and consequently a more intense 

 light, though at the cost of a larger amount of the burning 

 material. 



In this lamp the heated air and products of combustion 

 pass through a cylindrical opening in the reservoir, which 

 is placed directly above the lamp, the opening in it forming 

 as it were a prolongation of the chimney, thus not only pre- 

 venting the oil from freezing in the coldest weather, but 

 supplying it to the burner at the temperature best adapted 

 for perfect combustion. 



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