By turning this gallery _g_ (see figs. 33 and 36), therefore, motion is given to the tube, with its knobs _n_ _n_, whose vertical slit _a_ (while it holds the external peg of the wick-holder, and also turns it round along with it) permits that peg _a_ to slide upwards or downwards, and thus the wick-holder rises or falls, according as its own internal peg moves up or down the spiral groove in the tube S. In fig. 32, C shews the glass chimney resting on the gallery _g g_. [Illustration: Fig. 33.] [Illustration: Fig. 34.] [Illustration: Fig. 35.] [Illustration: Fig. 36.] ~Flowing of the Lamp.~ An important point in the economy of the Argand lamp, is the level at which the outlet for the oil, in its passage from the fountain to the burner, should be cut. The cutting of this hole (generally called the _flow-hole_) in the pipe is termed the _flowing_ of the lamp, and is commonly done by successive trials, until the oil stands at the proper level of the burner, before the wick is put in. A more ready and accurate method of accomplishing this object and at once determining the level at which the _flow-hole_ should be cut, was introduced by Mr JAMES MURDOCH, the Foreman of Lightroom Repairs to the Scotch Board, and is generally employed in the Northern Lighthouses. Its nature will be readily understood by a reference to the accompanying diagram, No. 37: [Illustration: Fig. 37.] The _hatched_ surface represents a metallic ruler, with a spirit-level at L; C is the cup in which the bottom of the fountain _f_ (shewn in dotted lines) rests. When the fountain is removed, and the ruler rests on the edge of the cup C, the screw at A is used to adjust the level at L; and a gauge GG is allowed to fall until a notch in it at _x′_ rests on the outer tube of the burner F; the pinching-screw B retains this ruler in its place, and the point _x′_ indicates the level at which the oil should stand in the burner. The level line _x′ x_ indicates the level on which the top of the _flow-hole_ H should be cut in the fountain-tube, which is shewn in dotted lines within the outer tube, or _body_ of the lamp. In other words, _y′ x′_ measures the level at which the oil should stand in the burner _below_ the lower edge of the metallic ruler, while the corresponding line _y x_, at the opposite end, shews the level of the top of the _flow-hole_ H, below the edge of the cup C. The gauge GG applied to that point of the fountain which coincides with the edge of the cup (so that _y′_ coincides with _y_) measures the length _yx_ = _y′ x′_; and a _set-square_ applied at _x_ gives the position of H on the fountain-tube. The round dot at _a_ shews the position of the air-hole in the body of the lamp, which establishes a connection between the external air and the surface of the oil. The rods SS′ shew the sliding gear (described as _d_ and _f_, page 221), and are only introduced to identify this diagram with those of the fountain and burner which have preceded it.