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66 THE BOOK OF THE LANTERN. regulated—a very difficult matter — by the screw plug on the nozzles of the bottles. 3. The operator at the lantern, even if he succeed in thus procuring the delivery of the proper amount of oxygen and hydrogen respectively, must stoop down to do so, and, for the time, neglect other duties. 4. The pressure is constant; and, therefore, if a double or treble light is suddenly wanted— as in the case of biunial and triple lanterns —the amount of gas measured out for one light, must serve for two or three, as the case may be— and all suffer. 5. As the bottles gradually empty, the pressure sinks; and, therefore, the screw-plugs have to be opened several times during an evening's work. Every time this becomes necessary, there is a likely chance of too much being turned on, and the tubes being blown off. It is, therefore, seen that, although bags are troublesome, bottles possess many disadvantages which would make any careful operator pause before he adopted them. These disadvantages, however, entirely disappear when the bottles are used with regulators. The action of the regulator can be understood without much difficulty by reference to the annexed sectional diagram, fig. 28. The screw-thread d* at the bottom of the drawing, is where the casting D fits upou the bottle of compressed gas ; d l is the delivery tube, governed by the stop-cock d 2 . B is a base-plate supporting the most im- portant part of the apparatus, and E E standing upon it is merely a casing to protect the enclosed part from injury. A A are bellows made of the finest rubber, and of a form not unlike the bellows of a camera, only that it is circular. The top of the bellows is heavily weighted, jso that its