AIR PUMP 221 allows a connection between T and R or be- tween T and the glass ball P; when, in the latter condition, the vessel T is raised so high that the mercury enters the ball P, then of course all air is expelled from T. When, now, the cock P is turned so as to establish the connec- tion between T and R (R being joined to the vessel to be exhausted), and if at the same time the vessel B is low- ered so that the surface of the mercury in it is 30 inches be- low T, the mercury in the latter will descend and fill B, while the vacuum in T will withdraw the air from the vessel to be exhausted. The cock O P is then turned again and B raised, which will ex- pel the air through P, and the operation is repeated. Ba- bo has modified this apparatus u by substituting valves for the stopcock, while Poggendorff has contrived a very useful combination of this instrument with the ordinary air pump. Bun- sen uses falling water to carry the surround- ing air with it (see ADHESION OF LIQUIDS TO GASES), and in this way produces a steady ex- haustion of 'air or vapor from vessels requir- ing such constant removal. It consists of a wide glass tube D, in which a narrower tube FIG. 6. Bunsen's Air Suction Pump. reaches downward to N", connected at the top by a well fitting cork M. Water is carried in by a side branch C, connected by means of an india-rubber tube B, closed by a spring H, with a tube A drawing water from a reservoir. The current of this water going down in the tube D around the inner tube, will produce a suction, drawing the air from T and S, and from any vessel connected with S. To in- crease the effect, the wide tube D is connected below with a lead tube F, which reaches 20 to 30 feet down ; and this long descending column of water acts like a powerful continuous pis- ton. The amount of rarefaction is ascer- tained by the difference in the height of the mercurial columns in the syphon barometer P Q. This apparatus is used for driving liquids through filters by atmospheric pressure, for drying in vacuo, &c. Some experimenters, in order to economize water, reverse the opera- tions of the tubes and pass the water out of the central narrow tube, while the suction takes place through the wide tube, in which case the water is passed in at S, while the exhaus- tion takes place by A; in this case the ba- rometer P Q is also connected with A. One FIG. 7. Doyle and Martin's Kotary Air Pump. of the most ingenious inventions of this kind is the rotary air pump of Doyle and Martin of New York. It consists of a wheel of which the rim is a hollow tube, filled in its lower portion with mercury, c c fig. 7 ; this mer- cury performs the function of a perfectly fit- ting piston, with a minimum of friction. When the wheel is revolved rapidly around its axis, the heavy mercury remains of course in its lower portion. The hollow rim possesses two or more stopcocks, v w, which in one posi- tion allow the mercury to pass, as represented below at w, and in another position close the communication between the two sides, and bring each in connection with one of the hol- low spokes, as seen at v in the top ; the position of these cocks is regulated by the levers m & and n g h, worked by the stationary grooved cam . If, now, the whole wheel, revolves rapidly in the direction of the arrows, the up- per cock, being closed, will compress the air at the left side and cause an exhaust at the right side, while the mercury remains below as the cock w is open ; the two curved spokes, marked " pressure pipe " and " suction pipe," will thus perform their respective functions, till the valve, having reached the mercury be-
