PNEUMATICS. 
Receiver, except about ^ 5 ^ part, when the 
other gauges do not shew a degree of ex- 
liaustion of more than two hundred times, 
and sometimes much less. 
When the receiver is placed upon the 
plate of the air-pump without exliausting 
it, it may be removed again with the ut- 
most facility, because there is a mass of air 
under it, that resists, by its elasticity, the 
pressure on the outside ; but exhaust the 
receiver, thus removing the counter pres- 
sure, and it will be held down to the plate 
by the weight of the air upon it. What 
the pressure of the air amounts to, is exaetly 
determined in the following manner : when 
the surface of a fluid is exposed to the air, 
it is pressed by the weight of the atmos- 
phere equally on every part, and conse- 
quently remains at rest. Bal; if the pres- 
sure be removed from any particular part, 
the fluid must yield in that part, and be 
forced out of its situation. 
Into the receiver A, (fig. 4), put a small 
vessel with quicksilver, or any other fluid, 
and through the collar of leathers at B, 
suspend a glass tube, hermetically sealed, 
over the smalt vessel. Having exhausted 
the receiver, let down the tube into the 
quicksilver, which will not rise into the 
tube as long as the receiver continues 
empty. But re-admit the air, and the 
quicksilver will immediately ascend. The 
reason of this is, that upon exhausting the 
receiver, the tube is likewise emptied of 
air ; and therefore, when it is immersed in 
the quicksilver, and the air re-admitted into 
the receiver, all the surface of the quick- 
silver is pressed upon by the air, except 
that portion whicli lies above the orifice of 
the tube : consequently, it must rise in the 
tube, and continue so to do, until the 
weight of the elevated quicksilver press as 
forcibly on that portion which lies beneath 
the tube, as the weight of the air does on 
every other equal portion without the 
tube. 
Take a common syringe of any kind, and 
having pushed the piston to the furthest end, 
immerse it into water ; then draw up the pis- 
ton, and the water will follow it. This is 
owing to the same cause as the last : when 
the piston is pulled up, the air is drawn out 
of the syringe with it, and the pressure of 
the atmosphere is removed from the part of 
the water immediately under it; conse- 
quently, the water is obliged to yield in 
fliat part to the pressure on the surface. It 
is upon this principle that all those pumps 
called sucking pumps act : the piston fitting 
tightly the inside of the barrel, by being 
raised up, removes the pressure of the at- 
mosphere from that part, and consequently 
the water is drawn up by the pressure upon 
the surface. See Hydiiaulics, and Pump. 
The effects arising from the weight and 
pressure of the atmosphere have been ab- 
surdly attributed to suction ; a word which 
ought to be exploded, as it conveys a false 
notion of the cause of these and similar 
phenomena. To prove that an exhausted 
receiver is held down by the pressure of 
the atmosphere, take one, open at top, and 
ground quite'flat, as A, (fig. 6 ), and covered 
with a brass plate, B, which has a brass rod 
passing through it, working in a collar of 
leather, so as to be air tight ; to this rod 
suspend a small receiver within the large 
one, a little way from the bottom ; place 
the receiver. A, upon the pump-plate, and 
exhaust it : it will now be fixed fast down ; 
but the small receiver may be pulled up or 
down with perfect ease, as it is itself ex- 
hausted, and all the air which surrounded 
it removed, consequently it cannot be ex- 
posed to any pressure j let, then, the small 
one down upon the plate, but not over the 
hole by which the air is extracted, and re- 
admit the air into the large receiver, which 
may then be removed ; it will be found, 
that the small one being itself exhausted, is 
held down fast by the air, which is now ad- 
mitted round the outside. If the large re- 
ceiver be again put over it and exhausted, 
the small one will be at liberty, and so on, 
as often as the experiment is repeated. 
Tliis effect cannot be accounted for upon 
any other principle than the pressure of the 
airj as the common idea of suction can 
have nothing to do in the case of the small 
receiver, which is fixed down merely by 
letting in the air round it. We ought, 
therefore, to attribute all those efiects 
which are vulgarly ascribed to suction, such 
as the raising of water by pumps, &c. to 
the weight and pressure of the atmos- 
phere. 
A square column of quicksilver, 29^ 
inches high, and an inch thick, weighs just 
15 pounds, consequently, the air presses 
with a weight equal to 15 pounds, upon 
every square inch of the Earth’s surfece ; 
and 144 times as much^ or 2,160 pounds, 
upon every square foot. The Earth’s surface 
contains, in round numbers, 200 , 000,000 
square miles ; and as every square mile 
contains 27,876,400 square feet, there must 
be 5,575,080,000,000,000 square feet on the 
Earth’s surface ; which number, multiplied 
