Also let me remark again upon the simplicity of the turbine as opposed to the other sort. The latter consists of a mass of moving and swaying rods and cranks, to work among which, as the engineers have to do, is a terrifying and nerve-racking experience. The turbine, on the other hand, has its only working part enclosed. It is difficult to tell, by looking at it, whether a turbine is at work or not, so silent and still is it, so self-contained. The reciprocating engine-room is noisy and full of turmoil: the turbine room is weirdly still by comparison. On the whole, too, it makes better use of the steam which it uses, but it has one decided drawback. It will not reverse, which the other type of engine does readily. This means that two turbines have to be coupled together, one with the blades so set that the steam drives it round correctly to produce motion ahead and the other set the opposite way so that it drives the vessel astern. The steam can be sent through either turbine at will and so motion can be obtained in either direction. Whichever turbine is in use the other revolves idly. Unfortunately it is impossible to make a turbine to go slowly and yet be efficient. Consequently, slow steamers cannot use turbines, but for warships, which are nearly all fast boats, it has almost displaced the older type of engine. The Curtiss turbine is different from the Parsons in that the steam encounters periodically, in its passage through, a partition perforated with funnel-shaped holes. Between the partitions it passes blades upon which it acts just as already described. The chief effect of this is to permit the machine being made of a rather more convenient shape and size. Other varieties of turbine are more or less combinations of the two ideas underlying these two. When we look at a locomotive in motion we always see steam coming out of the funnel, but we never see that in the case of a steamer. That is because all the energy of the steam is taken and used in the latter case, while in the former much valuable energy goes off up the funnel, making a puffing noise instead of doing useful work. On the steamship the steam is led not to the open air but to a vessel called a condenser the walls of which are kept cool by a continual circulation of cold water. The steam on entering the condenser at once collapses into water, leaving a vacuum. A pump called the "air-pump" removes the water (which was once steam) from the condenser and also any air which might get in, with the result that the engine is always discharging its steam into a vacuum. Thus to the pressure of the steam is added the suction of the vacuum. In turbine ships the cooling water for the condensers is circulated by powerful centrifugal pumps driven by subsidiary engines. The steam is obtained from boilers of that special variety known as "water-tube."