The most advantageous level of the flow-hole depends on many circumstances too obscure and complicated to admit of any systematic elucidation; and it is enough for all practical purposes, to know that the capillary powers of the wick, and the greater or less viscidity of the oil, are the chief circumstances which determine that level. Actual experience is the only sure guide to the best practice in this respect; and I therefore content myself with stating, that it is generally found that the sperm oil should stand in the empty burner at about ³⁄₈ inch below its top. For colza oil ²⁄₈ inch is sufficient. In summer, owing to the oil being more fluid, there is sometimes a tendency to overflow the burner; but any inconvenience arising from it is avoided by the plan adopted in the Northern Lights, of shutting off the oil (by means of the apparatus already alluded to on p. 222) about fifteen minutes before extinguishing the lights in the morning. [Illustration: Fig. 38.] The arrangement for cutting off the oil is very simple, as will be seen from the annexed diagram (fig. 38), in which F is the fountain, T the oil-tube leading to the burner, and V the _flow-hole_, with its sliding valve. By turning the handle H one quadrant of the circle, the whole fountain F and tube T turn round their vertical axis, while the valve V, which rests in a notch in the cup of the lamp, remains still, and sliding over T, opens the _flow-hole_. S is the screw-plug which retains the oil in the fountain, and which is unscrewed and removed when the fountain is to be filled. ~Placing the Lamp in the Focus.~ [Illustration: Fig. 39.] [Illustration: Fig. 40.]