HYDRAULICS. 
through the centre of the bottom, as seen in 
fig. 6. Thus, when the outlet is stopped by 
means of a finger applied thereto, the cup 
may be offered, quite full, to the person on 
whom the joke is to be practiced, observ- 
ing that the syphon will not act until the 
liquor in the cup exceeds the level of its 
bend, when the whole will be drawn through 
the tube. This whimsical contrivance is 
rendered yet more diverting by having the 
syphon so contrived, that its action may 
commence only when the cup is inclined a 
little, as is usual when a person is about to 
drink ; and if only a small flower, &c. be 
at the bottom of thevessel, appearing merely 
as an ornament, but allowing the liquor to 
pass under its petals, &c. into a tube made 
through one of two handles, and brought 
under the. bottom. 
Many springs are derived from natural 
syphons, existing in the sides of mountains, 
&cc. at various depths, and to various ex- 
tents. Some springs, situated on the tops 
of hills, near to larger ones, supply water 
all the year, others only periodically ; when 
they usually flow in profusion. In either 
case the ignorant multitude rarely attribute 
the supply to the proper cause. We shall de- 
monstrate from whence it originates. 
When various caverns, in which water is 
either pent up or received, lay in a regular 
descent, one below the other, the water 
will naturally pass from one to the other, 
and cause a regular flow, more or less abun- 
dant, according as the source may be more 
or less abundantly supplied. If the soil 
through which it passes be close and reten- 
tive, the water will then be occasionally 
raised, as well as lowered, in proportion to 
the weight of the incumbent fluid, and 
will rise, if so guided by the channel 
through which it pdsses, even to the 
height of the source, as may be proved 
by what has already been shewn in fig. 2. 
Thus, after various changes of altitude, the 
fluid may escape at any height not above 
that source ; or it may be carried away to 
any depth. The place where it issues forth 
is called a spring. Fig. 7, exhibits such a 
current, which we will suppose to have a 
perpetual supply. 
But; the intermitting spring may also have 
a regular supply. This is occasioned by the 
existence of caverns connected by syphons, 
as we may see by reference to fig. 8, where 
A is the source, b b the channel ; B is a 
cavern, which by means of the arch, or ris- 
ing channel, c c, becomes a syphon leading 
ing into D. It is obvious that, in the first 
instance, the water must, after filling B, 
rise in the channel, b b, as tn be above the 
greatest height of c c, to cause its passing 
off into E, and thence ad libitum. Now the 
channel, c c, being of greater diameter than 
the channel, b b, when the former com- 
mences its operation, it will discharge more 
than the latter can supply, so as to keep up 
the discharge from cc ; therefore, after B 
has been exhausted so far as to allow air to 
pass from it into cc, a certain quantity in 
that channel, which has not gained the sum- 
mit, will recede into B, and the water 
must again rise to the height in b b, which 
shall cause it to flow over the summit of 
c c, before the spring can again appear to 
be supplied. Yet the flow from the source 
was never diminished. 
The existence, or otherwise, of a vacuum, 
or void space, was long agitated, and that too 
with no small degree of acrimony, among 
the philosophers of old ; and we may say of a 
date by no means ancient. Common sense 
should have told us, what experience so 
amply proves, that where one body or ele- 
ment retires, another must supply its place, 
else the whole creation would inevitably be 
torn asunder. It is, indeed, well known, 
that the elasticity of the air, which could be 
rarified ad infinitum, if we had the means of 
effecting the process, enables it to occupy 
large spaces on emergency, or to contract 
within the narrowest bounds. See Pneu- 
matics. Under ordinary circumstances, 
however, we consider the air as being of a 
particular standard, namely, that a column 
ascending to the summit of our atmosphere, 
corresponds in weight with a column of 
water of thirty three feet in height, allow- 
ing the bases, i. e. of the air, and of -the 
water to be equal. Thus we find that 
where the air is withdrawn, by means of 
suckers, pistons, valves, &c. from within a 
pipe, of which the lowest part is immersed 
in the water contained in a well, &c. the 
fluid will rise to the height of thirty-three 
feet within the pipe, supplying the place of 
the air thus withdrawn. This is effected by 
the pressure of the atmosphere on the sur- 
face of the water ; whereby it is forced 
into the space formerly occupied by the 
air. Generally speaking, it is not a sudden 
operation ; for unless the well be very shal- 
low, it will require many 'strokes of a pump 
to withdraw so much air as may so far rarify 
the residue, within the pipe, as to allow the 
water to rise thirty-three feet above its le- 
vel. This is the greatest height to which 
water can be induced by a sucking pump* 
