PNEUBIATICS. 
side of its sails move against the air when 
it turns round. In each axle is a fine pin 
near the middle of the frame, which goes 
quite through the axle, and stands out a lit- 
tle on each side of it : under these pins a 
slider may be may be made to bear, and so 
hinder the vanes from going, when a strong 
spring is set or bent against the opposite 
ends of the pins. 
Having set this machine upon the pump- 
plate, draw up a slider, and set the spring 
at bend on the opposite ends of the pins : 
then push down the slider, and the spring, 
acting equally strong upon each mill, will 
set them both a-going with equal forces 
and velocities ; but the first will run much 
longer than the last, because the air makes 
much less resistance against the edges of’ 
its sails than against the sides of the other. 
Draw up the slider again, and set the 
spring upon the pins as before ; then cover 
the machine with the receiver upon the 
pump-plate ; and having exhausted the re- 
ceiver of air, push down the wire (through 
the collar of leathers in the neck) upon the 
slider ; which will disengage it from the 
pins, and allow the vanes to turn round by 
the impulse of the spring : and as there is 
no, air in the receiver to make any sensible 
resistance against them, they will both move 
a considerable time longer than they did in 
the open airj and the moment that one 
stops, the other will do so too. This shows 
that air resists bodies in motion, and that 
equal bodies meet with different degrees of 
resistance, according as they present greater 
or less surfaces to the air. 
Take a tall receiver, covered at top by 
a brass plate, through which works a rod 
in a collar of leathers, and to the bottom 
of which there is a particular contrivance 
for supporting a guinea and a feather, and 
for letting them drop at the same instant. 
If they are let fall while the receiver is full 
of air, the guinea will fall much quicker 
than the. feather; but if the receiver be first 
exhausted ; it will be found that they both 
arrive at the bottom at the same instant, 
which proves that all bodies would fall to 
the ground with the same velocity, if it 
were not for the resistance of the air, which 
impedes most the motion of those bodies 
that have the least momentum. In this 
experiment the observers ought not to look 
at the top, but at the bottom of the re- 
ceiver, otherwise, on account of the quick- 
ness of their motion, they will not be able 
to see whether the guinea and feather fall 
at the same instant. 
Take a receiver, having a brass cap fitted 
to the top with a hole in it ; fit one end of 
a dry hazel branch, about an inch long, 
tight into the hole, and the other end tight 
into a hole quite through the bottom of a 
small wooden cup ; then pour some quick- 
silver into the cup, and exhaust the receiver 
of air, and the pressure Of the outward air 
on the surface of the quicksilver will force 
it through the pores of the hazel, from 
whence it will descend in a beautiful show- 
er, into a glass cup placed under the re- 
ceiver to catch it. 
Join the two brass hemispherical cups, A 
and B, together, (fig. 8) with a wet leather 
between them, having a hole in the middle of 
it ; then having screwed otf the handle at G, 
screw both the hemispheres put together 
into the pump-plate, and turn the cock E, 
so that the pipe may be open all the way 
into the cavity of the hemispheres ; then 
exhaust the air out of them, and turn the 
cock ; unscrew the hemisphere^ from the 
pump, and having put on the handle C, 
let two strong men try to pull the hemis-* 
pheres asunder by the rings, which they 
will find hard to do ; for if the diameter of 
the hemispheres be four inches, they will 
be pressed together by the external air 
with a force equal to 190 pounds ; and to 
show that it is the pressure of the air that 
keeps them together, hang them by either 
of the rings upon the hook of a wire in the 
receiver of the air-pump, and, upon ex- 
hausting the air out of the receiver, they 
will fall asunder of themselves. 
Set a square phial upon tlie pump-plate, 
and having covered, it with a wire cage, 
put a close receiver over it, and exhaust 
the air out of the receiver; in doing which, 
the air will also make its way out of the 
phial, tiuougli a small valve in its neck. 
When the air is exhausted, turn the cock 
below the plate to re-adinit the air into the 
receiver ; and as it cannot get into the phial 
again, because of the valve, the phial will 
be broken into some thousands of pieces by 
the pressure of the air upon it. Had the 
phial been of a round form, it would have 
sustained this pressure, like an arch, with- 
out breaking; but as its sides are flat, it 
cannot. 
Let a large piece of cork be suspended 
by a thread at one end of a balance, and 
counterpoised by a leaden weight, suspend- 
ed in the same manner, at the other. Let 
this balance be bung to the inside of the 
top of a large receiver ; which being set on 
the pump, and the air exhausted, the cork 
