240 Of a Suction and Forcing Air Pump. 
The mechanism by which the piston is moved, is too obvious to 
need description. ‘There is, however, a peculiarity in the construc- 
tion of the piston rod, which is of great utility. The rod is hollow, 
having been sufficiently reduced in diameter from a piece of gun 
barrel by the wire drawing process. The bore of this hollow rod 
is occupied by a solid rod, which extends from the metallic disk, at 
the farther end of the piston, to the rack. To the other disk, the 
hollow rod is fastened. ‘The leather packing between the disks, 
being turned in the lathe so as to fit the calibre of the chamber ac- 
curately, is made more or less tight by the action of a screw just 
above the rack. Hence the pressure may be regulated without 
taking the pump apart, which is always troublesome, and at some 
periods impracticable within the time at command. 
With respect to the efficacy of this pump, satisfactory proof was 
given some time since, at the Franklin Institute, when it raised the 
mercury very near to the height of that in the Torricellian tube. 
Having been in possession for many years of an elegant air pump 
with glass chambers furnished by Pixii, we have been induced to 
give the preference to the new instrument, in all cases where a per- 
fect exhaustion has been desirable. 
Of the three valve cocks, one usually communicates with a gage ; 
since, instead of an instrument of that nature permanently associated 
with the pump, and which is subjected to exhaustion by means of a 
lateral communication with the perforation leading to the cavity of 
the receiver, I employ a movable barometer gage, which is made to 
communicate with the receiver directly. ‘The operator is thus ena- 
bled to observe the quantity of gas in the receiver, after the com- 
munication with the air pump is arrested by closing the valve cock 
through which it was established. An exemplification of this method 
of manipulating will be afforded by the apparatus and eudiometrical 
process, described in another article.* ~ 
* See Vol. xxxii. p. 280, of this Journal. 
