74-2 MECHANICS. 
tal ; if of glafs, upon the top there h laid a brafs plate 
with a ftop-cock a, having under it a prepared piece of 
leather to make it air-tight, and alfo a like plate at the 
bottom. Into the cock at x there is fcrewed a fyringe EF, 
having a fucker n, which is moved by a handle at H ; at 
ui there is a valve which opens downwards, and at o there 
is an orifice. Now let the fucker be drawn up above the 
orifice o, and both the barrel of the fyringe and the re¬ 
ceiver be filled with air in its natural ftate: then, upon 
forcing down the fucker, the air opens the valve at w, and 
a barrel of common air is forcer! into the receiver. Upon 
raifing again the fucker n, a vacuum is left under it, the 
valve preventing the air from returning; and, when the 
fucker gets above o, the -air will immediately rufii in and 
fill the barrel; thus upon every defeent of the fucker, you 
force into the receiver a barrel of common ajr, and confe- 
quently you condenfe the air in the receiver. After the 
receiver is charged, the flop-cock at a may be turned to 
prevent the return of the air, and the fyringe may be taken 
off, and any other apparatus may be fcrewed on for expe¬ 
riments with the condenfed air in the receiver. 
Prop. XLII. If b reprefent the capacity of the barrel of the 
fyringe, and r that of the receiver , then, after t defeats of the 
fucker, the denfity of the air in the receiver will be to the denjity 
at frf, in the ratio of r+tb : r.—For, the quantity of air 
at firlt may be reprefented by r, and, after t defeents of the 
lucker, a quantity reprefented by t b will be forced into 
the receiver, and therefore the whole quantity in it will 
be r+tb. Hence, The denfity after t defeents : the den¬ 
fity at firlt :: r+tb : r. Hence, the denfities after any 
number of fuccellive defeents, are in arithmetic progrefiion. 
If a be, fig. 4.7, be a glafs tube with the end at a open, 
and the other end hermetically fealed, and a fmall quan¬ 
tity of mercury put in fo as to leave the air in dc in its 
natural ftate ; then, if this be put into the receiver with 
the part be horizontal, and the air be condenfed, the con¬ 
denfed air prefling on the mercury will force it towards c, 
and the air in dc will continue of the fame denfity as that 
in the receiver. Now, as the denfity is inverfely as the 
fipace occupied by the fame quantity, the denfity in dc, 
and confequently in the receiver, is inverfely as dc-, when 
therefore dc is diminifhed until it be n times lefs than it 
was at firft, the denfity will be increafed ?2 times. Hence, 
as the denfity, after any number of fucceflive turns, in- 
creafes in arithmetic progrefiion, the reciprocals of the 
ipaces will be in arithmetic progrefiion, and therefore the 
ipaces themfelves will decreafe in mufical progrefiion. 
This inftrument is called a gage. 
A bell in condenfed air founds louder than in air in its 
natural ftate. Fire-engines, air-guns, artificial fountains, 
fome kinds of forcing-pumps, &c. att by condenfed air. 
The receiver, or veiTel containing the condenfed air, 
fliould be made very ftrong, to bear the force of the air’s 
elafticity thus increafed; for which reafon it is commonly 
made of brafs. When glafs is ufed, it will not fuftain fo 
vreat a condenfation of air ; but the experiments will be 
rendered more entertaining, as the effect of the condenfed 
air upon any fubjeft put within the receiver may be viewed 
through the glafs. 
II. Of Water-Pumps and Siphons. 
It would be an entertaining and not uninftrufifive piece 
of information to learn the progreftive fteps by which the 
ingenuity of man has invented the various methods of 
raifing water. A pump muft be confidered as the laft ftep 
of this progrefs. Common as it is, and overlooked even 
by the curious, it is a very abftrufeand refined invention. 
Nothing like it has been found in any of the rude nations 
whom the reitlefs fpirit of the Europeans has difeovered, 
either in the new continent of America, or the iflands of 
the Pacific Ocean. Nay, it was unknown in the culti¬ 
vated empire of China at the time of our arrival there by 
fea ; and it is ftill a rarity every-where in Alia, in places 
unfrequented by the Europeans. It does not appear to 
have been known by the Greeks and Romans in early 
times ; and perhaps it came from Alexandria, where phy- 
fical and mathematical fcience was much cultivated by the 
Greek f'chool under the prote&ion of the Ptolemies. The 
performances of Ctefibius and Hero are fpoken of by piiny 
and Vitruvius as curious novelties. It is perhaps not 
difficult to trace the fteps by which thofe mechanicians 
were'led to the invention. The - Egyptian wheel was a 
common machine all over Afia, and is Hill in ufe in the 
remoteft corners, and was brought by the Saracens into 
Spain, where it is ftiil verp common under its ancient 
name noria. The Danish miffionaries found in a remote 
village inth© kingdom of Siam the immediate offspring of 
the noria (Ltllres Edifiantes ct CurieufsJ. It was a wheel 
turned by an afs, and carrying round, not a firing cf 
earl hen pots, but a firing of wil'ps of hay, which it drew 
through a wooden trunk. This rude chain-pump was in 
frequent ufe for watering the rice-fields. It is highly pro¬ 
bable that it is of great antiquity, although we do not re- 
colleft its being mentioned by any of the Greek or Roman 
writers. The Arabs and Indians were nothing lefs than 
innovators; and we may fuppofe with great fafety, that 
what arts we now find among them they poflefled in very 
remote periods. Now the ftep from this to the pump is 
but fhort, though it is nice and refined ; and, for the in¬ 
vention of the pump, as well as of the clock, it appears 
that we are indebted to Ctefibius of Alexandria. See the 
article Horology, vol. x. p". 303. but it fhould be ob- 
ferved, that Dr. Blair places this great rnan at leaft a hun¬ 
dred years later than the date we have affigned to hint in 
that article, ftating him to have flourifhed in the time of 
Ptolemy Phyfcon,as we have it under the word Ctesieius, 
vol. v. 
The original purnp of Ctefibius is reprefented at fig.4-S. 
A B C D is a brafs cylinder with a valve L in its bottom. 
It is furnifhed with a pifton M K made of green wood, fo 
as not to fwell in water, and adjufted to the bore of the 
cylinder by the interpofition of a ring of leather. The 
tube Cl connects the cylinder ABCD with another tube 
NH, the bottom of which is furnifhed with a valve I open¬ 
ing upwards. Now, when the extremity D C of the cy¬ 
linder is iminerfed in wafer, and the pifton MK elevated, 
the prefiure of the water upon the valve L from below will 
be proportioned to the depth below thelurrace; the valve 
will therefore open, and admit the water into the cylinder. 
But, 4 when the pifton is deprefled, it will force the water 
info the tube Cl, and through the valve I into the tube NH. 
As foon as the portion of water that was admitted into 
the cylinder ABCD, is thus impelled into the tube NH, 
the valve I will clofe. A fecond elevation of the pifton 
will admit another quantity of fluid into the cylinder, and 
a fecond depreffion will force it into the tube NH; fo that, 
by continuing the motion of the pifton, the water may 
be elevated to any altitude in the tube. From this pump 
of Ctefibius are derived the three kinds of pumps now 
commonly ufed, the fucking, the lifting, and the forcing, 
pump. Of thefe we fhall give merely fuch a general de- 
l'cription of their conftruftion as will enable the iiudent to 
unaerftand the principles on which their operation depends. 
1. The fucking-pump is reprefented at fig. 49. The 
pifton is a body ABCD oP circular bafe, (figs. 49, 50, 51.) 
which may be moved through the interior part of the 
tube or body of the pump, filling it exactly as it moves 
along. The fucker E is moveable about a joint in fuch a 
manner as either to permit or fo prevent the paflage of the 
water, according as it preftes upwards or downwards. In 
figs. 49, 50. there are likewife fuckers in the piftons. 
FGHK (figs. 49, 51.) is another tube joined to the body 
of the pump, and is generally called the pipe, or fucking- 
pipe; its lower extremity is immerfed in the water, of 
which we fuppofe R S is the horizontal furface. In this 
pump, if we fuppofe a power P, fig. 49. applied to the han¬ 
dle of the pifton fo as to raife it from I to C, the air con¬ 
tained in the fpace DVKHGFC tends by its ipring to oc¬ 
cupy the fpace that the pifton leaves void : it therefore 
forces up the valve E, and enters into the body of the 
pump. 
