DISPLACEMENT GASHOLDERS.--An excursion may here be made for the purpose of studying the action of a displacement holder, which in its most elementary form is shown at C. It consists of an upright vessel open at the top, and divided horizontally into two equal portions by a partition, through which a pipe descends to the bottom of the lower half. At the top of the closed lower compartment a tube is fixed, by means of which gas can be introduced below the partition. While the cock is open to the air, water is poured in at the open top till the lower compartment is completely full, and the level of the liquid is at _l_. If now, gas is driven in through the side tube, the water is forced downwards in the lower half, up through the depending pipe till it begins to fill the upper half of the holder, and finally the upper half is full of water and the lower half of gas an shown by the levels _l'_ and _l"_. But the force necessary to introduce gas into such an apparatus, which conversely is equal to the force with which the apparatus strives to expel its gaseous contents, measured in inches of water, is the distance at any moment between the levels _l'_ and _l"_; and as these are always varying, the effective pressure needed to fill the apparatus, or the effective pressure given by the apparatus, may range from zero to a few inches less than the total height of the whole holder. A displacement holder, accordingly, may be used either to store a varying quantity of gas, or to give a steady pressure just above or just below a certain desired figure; but it will not serve both purposes. If it is employed as a holder, it in useless as a governor or pressure regulator; if it is used as a pressure regulator, it can only hold a certain fixed volume of gas. The rising holder, which is shown at A^1 in Fig. 1 (neglecting the pin X, &c.) serves both purposes simultaneously; whether nearly full or nearly empty, it gives a constant pressure--a pressure solely dependent upon its effective weight, which may be increased by loading its crown or decreased by supporting it on counterpoises to any extent that may be required. As the bell of a rising holder moves, it must be provided with suitable guides to keep its path vertical; these guides being arranged symmetrically around its circumference and carried by the tank walls. A fixed control rod attached to the tank over which a tube fastened to the bell slides telescope-fashion is sometimes adopted; but such an arrangement is in many respects less admirable than the former.