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90 OPTICAL PROJECTION sufficient to do so. It is most convenient to arrange a pair of cylinders upright in a little packing-case, which stands firmly on the floor behind the lantern, and is also convenient for carriage; but when one cylinder only is used, it is often laid flat on the table, if one is used as a stand for the lantern. It should, however, be tied to prevent rolling off. For a blow-through jet the pressure is better adjusted to only 5 or 6 inches. 49. Dangers of Compressed Gas.—The use of gas in cylinders was said by some persons to remove all danger. Experience has not quite borne this out, though what danger there may be, appears to have attached to the persons engaged in pumping it, and not to the consuming public. Years ago, a cylinder exploded at the Eoyal Institution whilst being filled, and examination showed that the intense heat caused by the rapid compression had ignited some particle of carbonaceous matter, and that this had in turn ignited the iron itself, which readily burns in oxygen. This is now guarded against. Another bottle burst when sent to have a * stuck' valve loosened, because the workman put it on the fire! No vessel would, of course, resist such an idiotic proceeding, which is scarcely likely to happen again. I have heard of no cylinder failing in England, whilst in the hands of the public; in testing, a certain portion of course do so, and are accordingly rejected. A pressure-gauge has burst, with serious, though not fatal results ; and it is safest to have such gauges well secured against, at all events, broken glass. The only practical danger is in mixture. A terrible and fatal explosion at Dublin, in the early part of 1889, was caused by oxygen being pumped into a hydrogen cylinder for an emergency, the cylinder on being sent back being filled up again with hydrogen. It was the vendor himself who was killed ; and he knew, and had been actually warned of, the state of affairs and the danger, running the risk rather than lose a few cubic feet of gas! And a similar accident—also