Another valuable improvement, first introduced by Ar gand, is the mechanism to elevate or depress the wick at pleasure above the tops of the tubes E, and thereby regu late the height of the flame. In Argarm's original lamps, the wick was raised up by a rack and pinion, but the more modern construction is shewn in Fig. 2. The exterior sur faco of the tube I:, Fig. 2. has a spiral groove or notch formed round it,and the cylindrical cotton wick G is stretch ed tight over a short piece of tube or ring r, which slides up and down upon the tube F, a small tooth projects on the inside of the ring r, and enters into the spiral groove. Now it is plain, that if the ring r is turned round, its tooth acting in the spiral groove will cause the tube to ascend and de scend, and also the wick which is attached to it. To give motion to the ring r from the outside, a moveable tube is placed within the tube E and F, and Meioses the ring r within it. On one side of this tube a notch is cut from top to bottom, and a second tooth, which projects from the out side of the ring, enters into this notch. The tube rises a little above the top of the external tube E of the burner, and has three small wires p fastened to it, which descend to the gallery I I, and ale fixed thereto so as to support it ; the same wires also fit the interior of the glass chimney, and prevent it from being overthrown. By turning the gallery I I round, the tube attached to it is made to turn round, and the projecting tooth of the ring r communicates motion to the ring also. As before mentionee, the interior tooth of the ring. acting in the spiral groove, moves the ring r and the wick up or down ; the notch in the side of the tube, which turns the ring, allows it to rise or fall with out communicating a similar !notion to that tube or gallery. R is a shade w hich surrounds the light and prevents its ac tion on the eyes. 'Ills is particularly useful for reading or writing, as it also reflects the light upon the paper.
The construction of the fountain reservoir for the Ar gotic] lamp, is also explained in Fig. 2. The oil reservoir A terminates at the lower end with a neck which screws into the oil cistern B. A hole n is made in the neck at one side for the oil to conic out ; but this cannot happen until the surface of the oil in the cistern B is drawn clown below the dotted line, so as to admit air to enter the opening, and then a drop of oil conies down. The air has free admission into the cistern B through a hole s. When the lamp is ex tinguished, the small knob or handle t is drawn up, which closes the hole n by the short tube o sliding over It, and pre vents the oil from being spilt if the lamp is inclined.
The fountain reservoir casts a very extensive shadow, and is therefore best adapted to be placed against a wall. It is a great advantage in lamps to have the oil reservoir situated beneath the burner, so that the light will not be in tercepted in any direction; and there are several ingenious methods of raising up the oil to a constant level for the ply of the wick.
Di-. Hook's semicylindrical counterpoise, described in our article HYDRODYNAMICS, Vol xi. p. 484, was intended to retain the oil of a lamp always at a constant height.
The hydropneumatic lamp is a small Hungarian ma chine or Chemnitz fountain ; see our article HyDRoNAM les. The pedestal of the lamp contains three oil cisterns, one above the other. The upper cistern is at the top of the pedestal, immediately beneath the burner or wick holder, and is made air tight ; it has a pipe ascending from the bottom of it to feed the burner with oil ; this is there fore the oil reservoir. The second cistern is placed beneath the former, and is open to the external air ; a pipe descends from it to the bottom of the third or lower cistern. The
latter is made impervious to air, and has a pipe ascending from the top of it to the top of the upper vessel or ieser Oir.
Suppose the two upper cisterns to be full of oil, and the lower one empty ; the oil in the second cistern behtg press ed upon by the air, the oil in it descends through the pipe to the bottom cistern, and enters therein ; but as the con tained air cannot escape from this vessel except through the air pipe to the upper vessel or reservoir, it ascends therein, and presses upon the surface of the oil so as to force it up the pipe to the burner, which is constructed, on Argand's plan, as in Fig. 2. The height to which the oil will rise in the space between the two tubes EF of the burner, is regulated by the height of the middle cistern above the lowest, because the pressure of that column of fluid is transferred by the medium of the air to elevate the oil out of the top cistt.rn into the burner.
When all the oil in the second cistern has descended to the lower cistern, the power of action wilt be exhausted, and roust be renovated by inverting the whole lamp, and then the oil from the bottom cistern runs back, and tne oil reservoir being again filled, the lamp is again ready to pro ceed. In lamps for common us.., the reservoir is made sufficiemly capacious for the consumption of one night ; but they have been made for halls and staircases of a sufficient size to hold oil for a month's consumption.
This lamp is called the French lamp, because they were first brought from France. But we have seen a letter from Mr. Watt to M. Argand, dated 1787, suggesting this plan in a form very little different from what is at present used ; and in consequence of this, M. Argand first made them in France.
We have lately found a figure of such a lamp in an Ita lian work, entitled Le Machine, by Branca of Rome, publish ed in 1629.
The hydrostatic lamp, first described by Dr. St. Claire in the Philosophical Transactions, and afterwards unproved by Mr. heir, is shewn in section in Fig. 3. FF is the burner where the wick G is placed. This may be constructed up on Argand's plan, as we have already described, but the figure represents a flat wick G, with a rack and pinion for elevating and regulating its height above the nozzle. The oil reservoir is situated in the bottom of the pedestal at A, and is closed on all sides, except where a pipe a b ascends to the burner E ; and also another pipe, which communi cates with a small cistern R. The latter is open to the at mosphere, and is filled with a solution of salt and water, which fluid being of greater specific gravity then the oil, a column of it, which is equal in altitude to AB, will sustain a higher column of oil, viz. from A to the dotted line F, which is the level at which the oil will sand in the burner. The proportion of the column of water to that of the oil is generally as three to four. As the oil is diminished by burning, the height of the column of eil will be diminished, ;;lager balance the column of water, winch will therefore descend; but on roicrin% into the vessel A it will displace the oil from it, and before the two come again to an equilibrium, the water will have restored to the oil column three-fourths of that height n hich it had lost by waste. As this loss of weight is to be reckoned on the surface of the large reservoir it is very slow ; and from the above con trivance, the height of oil only partakes of one-fourth of it. Taerefore the height of the oil in the burner is kept suffi ciently constant to supply the flame very regularly.