496 WRITINGS OF JOSEPH HENRY. [1875 



lard oil, it is necessary (as has been said) that it should be 

 kept at a high temperature, and for this purpose the heat 

 of the draught of the lamp was passed through the centre 

 of the reservoir. 



Previous to the change in the illuminating material there 

 had been used in the light-house establishment three classes 

 of lamps, viz, the mechanical lamp for the first, second, and 

 third orders, and the moderator and fountain lamps for the 

 fourth, fifth, and sixth orders. 



In the mechanical lamp the oil was placed in a reservoir 

 below the burner and pumped up by means of clock-work. 

 This apparatus is of a complicated character, and is subject 

 to derangement. The valves must be renewed from time to 

 time and the clock-work cleaned. The proper performance 

 of these operations is beyond the skill of an ordinary keeper, 

 and requires the frequent service of a trained and expert 

 attendant. 



The moderator lamp is less complicated, and was invented 

 to obviate the difficulties just mentioned. In this the oil is 

 elevated by the descent of a heavy piston, and forced up 

 through a small conical hole, the flow being regulated by the 

 conical end of a wire, which is gradually withdrawn as the 

 weight descends, so as to give a less obstructed flow as the 

 hydrostatic pressure of the oil increases. From this arrange- 

 ment it takes its name of moderator lamp. This apparatus 

 however is liable to irregularity on account of derangement 

 of the supplying apparatus, the varying friction of the pack- 

 ing of the piston, as well as the change in the flow of the 

 oil, owing to its diminished liquidity on a reduction of its 

 temperature. • 



The reservoir of the fountain lamp consists in an air-tight 

 vessel, (usually cylindrical,) from the bottom of which descends 

 a tube, terminating at the open end in a small cup, from 

 which the burner is directly supplied with oil on the well- 

 known principle of the bird fountain, this vessel being filled 

 with oil by inverting it and pouring in the liquid through 

 the open end of the tube. It is then re-inverted and the end 

 of the tube inserted in the small cup below the level of the 



