414 Mr. F. D. Brown on the Maintenance of 



connected with the apparatus X, in which a constant pressure is 

 kept up. Sufficient mercury is poured into R to fill A B up to 

 the lower end of H when R is about half full ; C D is also filled 

 with mercury, which is let out by the tap M until the column 

 P S above the lower end of S S is equal to the difference be- 

 tween the required pressure and that of the atmosphere. If 

 now the air-pump be set to work 7 a partial vacuum will be 

 created in X and in the tubes A B and C D, the mercury will 

 rise in A B until it touches the lower end of H ; R is then placed 

 in such a position that the vertical distance between the lower 

 end of H and the surface of the mercury in R is equal to P S. 

 It is now evident that when the desired pressure is reached 

 the mercury will close up the orifice of H, thus stopping the 

 withdrawal of air ; while if the pressure is less than that re- 

 quired, more air will enter through the tube S S and bubble up 

 through the mercury, and thus a more or less constant pressure 

 will be maintained in X. 



This apparatus suffers from two defects : firsts the splash- 

 ing of the mercury as it is sucked up fl and then falls down 

 again, together with the bubbling of air up C D, renders the 

 pressure in X slightly variable ; secondly, it is only adapted 

 for pressures less than that of the atmosphere. 



Meyer's instrument has very recently been modified by Dr. 

 Otto Schumann and by W. Stadel and E. Hahn {Ann. Cliem. 

 Pharm. vol. cxcv. p. 218); the new form, although capable 

 of regulating pressures above an atmosphere, apparently with 

 tolerable accuracy, has a very limited range. 



The apparatus for maintaining constant pressures, which I 

 now wish to bring to the notice of the Society, consists of a ma- 

 nometer connected with which is an automatic arrangement for 

 governing the supply of air. The manometer (fig. 2) consists of 

 a tube A B having the form and dimensions shown in the figure; 

 the lateral tube C is connected with the vessel X, in which a 

 constant pressure is to be kept up ; the tube D is connected 

 with an air-pump or other contrivance for rarefying and com- 

 pressing air. The upper end of the tube A B is closed by an 

 india-rubber stopper, or, better, by a metal cap, through which 

 the rod E x passes air-tight; this rod is tipped with platinum 

 at its lower end. The lower end of A B is joined by means of 

 india-rubber tube to the tube F, of which the upper part has 

 the same diameter as that of A B ; this tube is fitted with 

 another cap and iron rod E 2 similar to the first ; but the cap 

 does not fit air-tight. The piece of wood which carries F 

 moves along the scale S S in a groove in the board to which 

 the whole is fixed. Lastly, the two rods E l7 E 2 are furnished 

 with binding-screws for copper wire, while a third binding- 



