is screwed to the pump, and put in communication with the exhausting-tube T, 

 fig. 4, above the stop-cock C, then the surface S, will be pressed by the elastic 

 force of the air in the receiver R, with which it communicates. So long as 

 that elastic force is capable of sustaining the column of mercury in the leg B, 

 above the level of the surface S, this instrument will give no indication of the 

 degree of rarefaction ; but when by the operation of the syringe, the air in the 

 receiver is so far exhausted that its elastic force is unable to sustain the mer- 

 curial column in B, A, above the level S, then the mercury will begin to fall 

 in the leg B, A, and the surface S will rise in the leg B, C. The column 

 suspended in the leg B, A, above the level S, will now be the exact measure 

 of the elastic force of the air in the receiver which sustains it. In this respect 

 the siphon-gauge must be regarded as a more direct measure of the elastic 

 force of the air in the receiver than the barometer-gauge. The latter, in fact, 

 measures not the elastic force of the air in the receiver, but the difference be- 

 tween that elastic force and the pressure of the atmosphere. 



To obtain the elastic force of the air in the receiver, it is necessary also to 

 ascertain the indications of the barometer. The siphon-gauge, however, gives 

 at once the pressure of the air in the receiver. 



The air-pump has been constructed from time to time in a great variety of 

 forms, the details of which it would not be proper to introduce into the present 

 treatise. The general principle in all is the same ; they differ from each other 

 chiefly in the construction of the piston and valves. 



In the form which has been above described, the air effects its escape from 

 the receiver at each stroke of the piston by opening the suction-valve V, fig. 4. 

 Now in whatever way this valve is constructed, it must require some deter- 

 minate force to raise it, and this force, in the case already described, is the 

 elastic force of the rarefied air remaining in the receiver. Thus the operation 

 of the machine is accomplished by the presence in the receiver of the very 

 agent which it is the object of the machine itself to remove, and from the very 

 construction of the instrument it must cease to act while yet air of a determinate 

 pressure remains in the receiver. 



This defect has been sometimes attempted to be removed by causing the 

 suction-valve to open, not by the pressure of the rarefied air, but by some me- 

 chanical means acted upon by the piston. Such contrivances, however, are 

 found to be attended with peculiar inconveniences which more than outweigh 

 their advantages. Probably the most simple and the best contrivance is one 

 in which the suction-valve is altogether dispensed with and the air passes 

 freely through the open tubes from the receiver to the pump-barrel. Let T, fig. 

 6, be the exhausting-tube which is carried from the receiver, and enters the 

 pump-barrel at a point distant from the bottom of the barrel by a space equal 

 to the thickness of the piston. The piston P, is a solid plug which moves air- 

 tight in the barrel, and is propelled by a polished cylindrical rod which slides 

 in an air-tight collar C, in the top of the cylinder, which in this case is closed. 

 A valve is placed in the top of the cylinder, which opens outward, and which 

 may be constructed in the same manner as the silk-valves already described. 

 When the piston descends it leaves a vacuum above it — the external air not 

 being allowed admission through the valve at the top ; and when the piston 

 arrives at the bottom of the barrel, it has passed the mouth of the exhausting- 

 tube T, and fills the space below it. The air in the receiver then expands 

 into the empty pump-barrel, and when the piston is raised, having passed the 

 mouth of the tube T, the air which has expanded into the barrel is confined 

 between the piston and the top, where, as the piston rises, it is condensed. 

 When in acquires sufficient elastic force it opens the valve at the top and is 

 discharged into the atmosphere. 



