It is obvious that in this machine the flow-of oil will be greatest when the piston has been newly wound up, and the spring is at its greatest tension. This inequality is regu lated, or the name of the lamp—by an extremely ingenious contriv ance, which narrows the passage for the oil when the pressure is strongest.
The introduction of mineral oils—known under the various names of paraffine oil, petroleum, kerosene,•naphtlia, shale oil, etc.—lias in a great measure superseded the use of animal and vegetable oils for lighting purposes. The great recommendation of the former is their cheapness. One great difficulty with the mineral oils at first was that, without careful preparation, they are apt to give off inflammable vapors at a low tem perature, which give rise to dangerous explosions. This has been obviated by processes of rectification which get rid of the lighter and more volatile ingredients. An oil that gives off an inflammable vapor at a temperature under 120° F. can hardly be considered sme. Paraffine oil from Boghead coal will not form an explosive mixture under 140° F. It is illegal to store or issue oil forming an inflammable mixture under 100° F. Another difficulty was to make the oil burn without smoke. The kind of lamp found to effect this purpose best was introduced into Great Britain from Germany about 1856. and, with minor improvements, the form is still adhered to. The body of the lamp is a globular-shaped reservoir of glass or stoneware for the oil, mounted on a foot or pedes tal; into this a brass wick-holder is screwed, the wick being raised or lowered by means of a rack and pinion. The peculiarity of the paraffine lamp is a dome-shaped cap sus rounding the wick-tube, and having a slit running across it, through which the flame issues. 'A long glass chimney rests on a ledge or gallery around the base of the cap; and
by perforations in the brass an air-chamber is formed below. The chimney causes a strong draught through this chamber, and the cap or dome deflects the current of air, and makes it impinge against the flame as it passes through the slit, thus producing per fect combustion and a white, brilliant light without smoke. The demand for these lamps has become so great, that the manufacture and sale of them forms an extensive business of itself. • A great drawback in the use of the common paraffine lamp is the expense and annoy ance attendant on the frequent breakage of the glass chimney. To obviate this, Rowatt & Son of Edinburgh have introduced their patent Anucapnic (smokeless) lamp, which dispenses with the glass chimney altogether. Instead of it, a second cap or dome is placed over the ordinary one. leaving a narrow space between the two. As the two cones get hot, a powerful draught iscreated, and two separate currents of air are directed against the flame,. one by the lower cap, as in the ordinary lamp, and the other from between the two caps. The result is perfect combustion, without a chimney. A large glass globe is used to protect the flame from currents of air, as well as to disperse and soften the light. Such a globe is also often used with the ordinary lamp in addition to the chimney, a flange for supporting it being added to the burner. Mineral oil thus burned furnishes a satisfactory light, rivaling gas in cheapness.