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68 THE BOOK OF THE LANTERN. between the space which is open between the valve/* and the valve seat d. It then rushes into the bellows above, which become gradually raised by its pressure. As the bellows rise the screw pillar F is quickly turned in its socket, and gives a slow motion to the screw below. The effect of this movement is to bring the valve f down on its seat d) and the supply of gas is cut off. But in practice the gas will be drawn off from the delivery tube d l so that the bellows will soon be compressed once more. As the bellows move downwards, the screw F acts in the reverse manner, so that the valve F is now raised, and a fresh supply of gas enters the bellows. In this way the bellows are constantly rising and falling. If but one lime-light is in use, and the delivery of the gas is therefore regular, the bellows will be almost stationary, for they will auto- matically adjust the valve F, so that just enough gas, and no more, will pass through the opening. But if two or three lights are in use, and sometimes only one, and per- haps immediately afterwards all three are requisitioned, then the little bellows will have a more lively time of it. In both cases the regulator will deliver the quantity of gas which happens at the moment to be required. Some time ago I contributed to The Camera, an illus- trated article showing how ? by the use of a small gas-bag, A, the outflow of gas from a cylinder might be regulated. The bag was in this case made to raise a lever as it filled, thus cutting off the gas supply from the bottle, until the hag was partially emptied, when the action was repeated. Shortly after the publication of this article a self-acting valve on the same principle was introduced. This valve