When lamps are required to give light in one direction, as when placed against a wall, or used as reading lamps, the fountain lamps, similar to that just described, are undoubtedly superior to all others, on account of the abundant and uniform supply of oil which they afford to the wick; but when a lamp is required to throw a light all round, as when placed on a table in the centre of a room, the fountain becomes objectionable on account of the shade it throws; in this case the burner is usually mounted upon a column, and is encircled by a hollow ring at a distance of some inches from it containing the oil which flows to the burner by two tubes, and in order that the level of the oil may not greatly vary, the ring is made as flat as possible. This ring also supports a ground glass shade, which, besides softening the light, by its peculiar form, so reflects and refracts the rays in every direction as nearly to prevent any shadow being cast by the reservoir; hence these lamps are termed 'sinumbra," or "shadowless lamps." But although the shadow thrown by such lamps is scarcely perceptible, the light is not equal to that of fountain lamps, owing to the supply of oil being neither so copious nor so uniform as in the latter; and they are also somewhat cumbrous and awkward to move, owing to the projection of the reservoir and glass shade, and to the centre of gravity of the lamp being carried so high up.
To remedy these defects has long been a favourite speculation with many persons, and generally every year one or more patents are taken out for lamps which are supplied with oil from a reservoir situated within the column which supports the burner. Few of these possess any claims to novelty, being most of them founded upon the principle of the Chremnitz fountain, in which a body of water descending through a given height forces a smaller quantity of water, contained in a close vessel, up to nearly an equal height by compressing the air above its surface. As illustrative of the principle we shall describe one or two of these lamps, although as we have already remarked, few of them exhibit much originality of thought. The figure on page 36 represents an arrangement for a lamp described in the Register of Arts as the invention of a correspondent in which the resemblance to the Chremnitz fountain will be at once recognised, a is a vessel containing water; b an oil vessel; c c a column and pedestal to support the lamp, closed at the top and bottom, and forming the air vessel; d an air tube in a, open at top and bottom; e a tube soldered into the top of the column c, and proceeding from the bottom of a to the bottom of cup f; g a similar tube soldered to c, and proceeding from b, the lower end descends a little way into the cup h; i is a glass tube ascending from the bottom of the cup h, through a tight joint, and branching at top into three capillary jets, forming the burner, and the tube I, which surrounds it, serves to receive any oil that may flow over; m and n are two plugs in the bottom of e and g.
To use the lamp proceed as follows: - Invert the lamp, withdraw the plugs m and n, fill a with water, and b with oil; then replace the plugs in the position shown in the drawing, and place the lamp on its base. The oil will now flow from b into h, until the mouth of the tube g is covered; at the same time the water flowing from a into f will compress the air in c, which, acting on the surface of the oil in h, will force it up the tube i to the burners; by this the oil in h will fall below the mouth of g, when a portion of the compressed air passes into b, displacing an equal bulk of oil; by these means the oil in h is always maintained at the same level as the mouth of g; the capacity of a is not equal to that of the base up to the level of the brim of the cup f; but it is clear that by means of the air-tube d, the height of the column of water will always be equal to the height of the lower opening of d, above the brim of f. To extinguish the lamp, push the plugs m and n into the necks of e and g, which stops the supply of oil.
The cup f is screwed into the bottom of c, and must be unscrewed to discharge the water in c, when the vessel becomes empty.
In the sketch on page 38, Fig. 1, represents a section of a lamp invented by Mr. Bright, of Bruton Street, which was exhibited at the National Repository, and which we have seen in use elsewhere, and it appeared to us to afford a strong and steady light. The principle is precisely similar to that of the one just described, but it is much more compact, and the general arrangement is better. This lamp, and the mode of its action, may be briefly described as follows: - The water vessel b is an inverted fountain, which empties itself into the air chamber c, through the pipe d; the air thus displaced is forced up the rising bent tube e into the oil vessel a, from whence, as it cannot escape, it presses upon the oil and forces it up the pipe f to the burner g. It will be seen that by this arrangement the two columns of oil and water will be constantly in equilibrio.
The last lamp upon this static principle which we shall notice, differs somewhat from the preceding, and possesses rather more novelty. It is represented in section on p. 38, Fig. 2, and the following is an extract from the inventor's description of it in the Register of Arts: - a a glass vessel forming the body of the lamp; surmounted by a glass column connected with a by the cork c, which fits tightly into each, and closed at top by the cork h. No. 2 a glass tube descending through the two corks h and c, to the bottom of the vessel a, and bent upwards again as far as g, it communicates with the column by the hole f, which may be closed by a sliding tube 5, and the latter be closed by the stopper k at its top. No. 1 a glass tube passing through the corks h and c, its lower end opening into a, and its upper connected by a stop cock with d, a glass vessel closed at top and bottom with corks. e a capillary tube descending half way down d. No. 3 a tube passing through h and c, and reaching to the bottom of a; it has two small openings into the column in its upper part, which may be closed by the stopper l. No. 4, (not seen in the section, but shown in Fig. 3, which is a plan of the tubes,) is a tube passing through h and c into the upper part of a.