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THE PHOTOGRAPHIC NEWS. EDITED BY T. C- HEPWORTH, F-C-S- Vol. XXXV. No. 1736.— December 11, 1891. CONTENTS. PAGE Pressure Gauges 841 Colour-Photometry, By Capt. Abney, C.B., R.E., D.C.L., F.R.S 842 Rodinal 844 Photography in Germany. By Hermann E. Gunther 844 The Measurement ef Lenses 845 Renovating the Camera. By H. C. Standage 846 Notes 848 PAGE Leeds International Photographic Exhibition 849 Allotropic Silver 850 New Lantern Attachment 851 Short Notices of Books 851 Patent Intelligence 852 Correspondence 852 Proceedings of Societies 853 Answers to Correspondents 856 PRESSURE GAUGES. We have recently had occasion to report a lantern disaster caused by the explosion of a gas-bag or bags— it is impossible to say whether one or both ignited—an accident which was fatal in one case, and which brought dire injury to many others. The judicial inquiry which followed this catastrophe was unsatisfactory in one respect, for the actual cause of the explosion was not made clear. The cause of explosions generally are often shrouded in mystery, because the chief actor, and the man responsible, has been among the sufferers; but in this case, happily, the operator was spared. He had apparently used every precaution in the prepara tion of the gas and its storage, and the jury very properly exonerated him from all blame. The occasion, however, has given experts and others an opportunity of decrying the use of bags as gas-holders, and advocat ing the employment of cylinders. The recommenda tion, perhaps, was unnecessary, for lanternists have long ago found out that the use of cylinders, besides being more convenient in every way, is actually more economical. In other words, the gas is cheaper to buy in the compressed state than it is to prepare at home, the saving of time alone being very considerable. We unhesitatingly endorse the recommendation to use cylinders, not only by reason of their many advan tages, but also because, as yet, there has been no re corded explosion of one of these gas receptacles while in use. They have certainly given way again and again under water pressure while being tested, and have also occasionally burst while being filled with gas. There is also recorded a case of an explosion of a cylin der while being experimented with, but there is reason to believe that in this case the apparatus was used in a very foolhardy and reprehensible manner. A well- tested, seamless cylinder of mild steel may be regarded as a far safer thing to work with than a bag. Under the system now adopted by the Brin Company of pro viding the H cylinder nozzle with a left-hand thread, so that it cannot possibly be connected with the oxygen supply pump at the works, and by giving the 0 cylinder a right-hand thread, so that it cannot be screwed to the H supply pump, it is evident the gases cannot be mixed, even if anyone attempted purposely to do so. But with a bag it is only too easy for an inexperienced or careless hand to mingle the two gases. The bags, for instance, may be a quarter full of gas after a performance—it is, of course, an economy to utilise that residue—and it is only too easy to fill up one of these bags with the wrong gas. It is idle to say that every precaution is taken when one bag is plainly marked with an “ H ” and the other with an “0,” for human nature is fallible, and will make mis takes. Custom begets carelessness, and even a trust worthy man may have his attention distracted at the moment when he is filling a bag, and may make a mis take which will lead to disaster. There is, therefore, no question that, by using a steel cylinder which can only be filled by a certain machine, which machine can only be attached to the cylinder holding the particular gas which it supplies, we avoid all chance of storing an explosive mixture. There is a danger in the use of a cylinder, but it is of another nature; it is confined to the operation of measuring the amount of gas contained therein by means of a pressure gauge. Such a form of gauge has been in use for many years for recording the pressure of steam boilers, and such an instrument forms a prominent feature of every steam.engine in the country, but we have never heard of one giving way. The average working pressure of a steam boiler is something like 100 pounds on the square inch; but in dealing with gas pressures in the modern form of cylinder, we have a different condition of things altogether, for the initial pressure of the gas in a fully charged vessel as it leaves the pumping station is 120 atmospheres, equal to 1,800 pounds on the square inch. We are, indeed, here dealing with something new, a state of things never contemplated by the writers of text-books; so that it is difficult to get any information on the subject. It is not, therefore, wholly remarkable that gases under