=185. Principle Upon Which Vacuum Cleaner Works.= The principle of a vacuum cleaner is that, thru suction, dust and dirt are drawn from the floor or other surfaces into some container. If the power of the cleaner is sufficient, it may pick up anything--but cleaners having a moderate amount of power are somewhat more discriminating. They do, however, remove the fine, greasy dirt that brooms, brushes and carpet sweepers fail to get. The coarser dirt and ravelings may be taken up by a carpet sweeper, with a brush, or picked up by hand. The brush is combined with the cleaner in many machines (Fig. 98). [Illustration: FIG. 98. Brush and vacuum cleaner combined.] =186. Different Kinds of Vacuum Cleaners.= There are cleaners with bellows, pumps or fans to draw in air and dirt. The ones with bellows in them work on the principle of a bellows which is reversed so that when the air is drawn in, it brings the dirt with it. The other kind works with a fan which draws or sucks air from the floor thru a nozzle into the machine. In the machine, the dust is filtered out of the air and collected in a pan. The machines with fans in them are mostly power machines, as the fan must revolve very rapidly. The hand machines are mostly of the pump and bellows types. Some are combined with the carpet sweeper, making two machines in one. With this device once going over the floor is sufficient for removing both coarse and fine dirt. The hand machines do not have as much power of suction as the power machines, but they do very satisfactory work. They are more effective than a carpet sweeper in removing dirt, but they do not get as much of it as the stationary cleaner. Removing the sharp grit from rugs and carpets lengthens the life of them so that the more grit a cleaner can remove without tearing the carpet, the more valuable it is. When the pump type is being used, the piston is drawn up, drawing with it air and the dirt which is present at the point from which the air comes. A cloth filters out the dust. The air escapes from the machine before the piston is lowered to draw in more air and dirt. If this were not true, the dust would be forced back as the piston was lowered. =187. Nozzle of Vacuum Cleaner.= The nozzle, or point of entry of air into the machine, is an important part of a vacuum cleaner. This is constructed so that it fits the surface from which the dirt is to be drawn, insuring the drawing up of dust as well as air. The dirt is drawn from only a few square inches of surface at one time. The thoroness and rapidity with which the dirt is removed depends upon the strength of the suction or the power of the machine. Thus, hand machines may have to be moved over a surface several times if it is very dirty in order to get all the dirt. Plain solid nozzles work best on carpets and other surfaces of similar kind. They are not effective on hard floors, but this is not essential, as dirt can easily be removed from smooth surfaces with a brush.