Those heights of flame can be obtained only by a careful adjustment of the heights of the wicks and the relative levels of the shoulder of the glass-chimney and the burner, together with a due proportion for the area of the opening of the iron-damper which surmounts it. The wicks must be gradually raised during the first hours of burning to the level of 7 millimètres (0·27 inch) above the burner, a height which they may only very rarely and but slightly exceed. By raising the shoulder of the glass-chimney the volume of the flame is increased; but, after a certain height is exceeded, the flame, on the other hand, becomes reddish, and its brilliancy is diminished. The height of the flame is decreased, and it becomes whiter by lowering the chimney. The chimney is lowered or raised by simply turning to the right or to the left the cylindric glass-holder in which it rests (see [Plate XXV.]). In regulating the flame, however, recourse is most frequently had to the use of the damper, by enlarging the opening of which the flame falls and becomes whiter and purer; while by diminishing its aperture, the contrary effect is produced. The area of the opening depends on the inclination of a circular disc capable of turning, vertically through a quadrant, on a slender axle of wire, which is commanded by the light-keeper by means of a fine cord which hangs from it to the table below. When the disc (see [fig. 84], p. 287) is in a horizontal plane the chimney is shut, when in a vertical plane it is open; and each intermediate inclination increases or decreases the aperture.
I need scarcely add, that in order to produce the proper effect of a system of lenses or refractors, the vertical axis of the flame should Position of flame in reference to focus of apparatus. coincide with their common axis; and it is further necessary, in order to bring the best portion of the flame into a suitable position with reference to the apparatus, that the top of the burner should be quite level, and should stand below the plane of the focus in the following proportions, viz.:—
| For | 1st order, | 28 | millimètres | = | 1·10 | inches. |
| ... | 2d order, | 26 | ... | = | 1·02 | ... |
| ... | 3d order, | 24 | ... | = | 0·95 | ... |
Fig. 85.
For the purpose of placing the lamp in the centre of the apparatus, a plumbet with a sharp point suspended in the axis of the apparatus, is used to indicate, by its apex, the place for the centre of the burner. The lamp is then raised or lowered as required by means of four adjusting screws Q at the bottom of its pedestal ([Plate XX.]); and the top of the burner is made horizontal by a spirit-level, the most convenient form of which is that of the spherical segment, which acts in every azimuth. Its application to this purpose is due, I believe, to M. Letourneau, the successor of M. François in the construction of dioptric apparatus at Paris. This level is shewn in the annexed figure ([fig. 85]), in which a b is the brass frame containing the level, and O the air-bubble; and e shews circles of equal altitudes engraved on the glass. After the first application of this level, the adjustment of the burner as to its central position is carefully repeated by means of a centre gauge (shewn at [fig. 90], p. 295), with reference to the vertex of each lens, or to many points on the internal surface of the refractors; and being found correct, the level is again applied to the top of the burner, to detect any deviation from horizontality that may have occurred during the process of adjusting it to the axis.
The lamp is subject to derangement, chiefly from the stiffness of the clack-valves for want of regular cleaning, bursting of the leathern valves of the oil-box, stiffness of the regulator, and the wearing of the bevelled gearing which gives motion to the connecting-rod that works the valves of the oil-pumps.
Working of the Pumps of the Lamp. The pumps of the lamp should raise, in a given time, four times the quantity of oil actually consumed by burning during that time. Their hourly produce should be,
| lb. avoirdupois. | ||
|---|---|---|
| For the lamp with | four wicks, | 6·615 |
| ...... | three wicks, | 4·410 |
| ...... | two wicks, | 1·675 |
This surplus of three times what is burned is necessary to prevent the wick from being carbonised too quickly; and it has been found quite sufficient for that purpose. The discharge from the pumps is, of course, regulated by changes in the angle of the fans of the regulator, or in the amount of the moving weight.