weight, which will then be too great for the preffure of the water round the tube upon the column of water below it.

Again, a piece of wood, however light, may be made to lie at the bottom of the water, by not fuffering any water to get under it. Thus, having two pieces of wood, planed quite flat and fimooth, so that no water may get between them, when they are put together; and cementing one of the pieces, as a b, to the bottom of the veffel A B, place the other piece upon it, and let it be held down by a stick, while the water is poured into the veffel; then, upon removing the ftick, the upper piece of wood will not rife from the lower one, being preffed down both by its own weight, and the weight of all the water above it, while the contrary preffure of the water upwards is kept off by the wood placed beneath it; but if the top piece of wood be raised ever so little at any part of its edge, fome water will get under it, which will be forced by all the weight of the water above, and will immediately prefs it upwards; and being lighter than its own bulk of water, it will float upon the surface of the water.

Το prove that all fluids weigh just as much in their own elements as they do in open air, put as much shot in a phial as, when corked, will make it fink in water; then let it be weighed, both in the air and in the water, and the weight in each case wrote down; then, as the phial hangs fufpended in water, and counterpoised by another weight, pull out the cork, that the water may run into it, when it will defcend and pull down that end of the beam. Next, put as much weight into the oppofite scale as will restore the equilibrium; which additional weight will be found to answer exactly to the additional weight of the phial, when it is again weighed in the air with the water in in

The velocity with which water spouts out of a hole, or through a tube in the fide or bottom of a veffel, is as the fquare root of the depth or diftance of the hole below the furface of the water. Therefore, in order to make double

the

the quantity of water run through one hole, as through another of the fame fize, it will require four times the preffure of water as the other hole hath; and confequently, it must be four times the depth of the other below the furface of the water; and for the fame reason, three times the quantity of a fluid running in the fame time through a hole of the fame fize, must run with three times the velocity; and confequently must be nine times as deep below the furface of the fluid. Thus, let C and g (fig. 12) be two pipes of equal diameters, fixed in the fide of the veffel A B, the pipe g being four times as deep below the furface of the water at b, as the pipe C is: while the water runs through these pipes, let more water be conftantly poured into the vessel, to keep the surface still at the fame height. Then, in the fame time that a pint of water flows through the pipe C, through the pipe g will flow one quart.

The horizontal distance to which a fluid will spout from an horizontal pipe, fixed in any part of the side of an upright veffel, below the furface of the fluid, is equal to twice the length of a perpendicular to the fide of the veffel, drawn from the mouth of the pipe to a femicircle, defcribed upon the altitude of the fluid; and therefore the fluid will spout to the greatest distance poffible from a pipe at the centre of the femicircle; becaufe a perpendicular to its diameter (which is fuppofed parallel to the fide of the veffel) drawn from that point, is the longeft that can poffibly be drawn from any part of the diameter to the circumference of the femicircle. Thus, if the veffel A B be full of water, the horizontal pipe D, in the middle of its fide, and the femicircle N deb be described upon D, as the centre with the radius Dd. The perpendicular D d, to the diameter Nb, is the longest that can be drawn from any part of the diameter to the circumference. And if the yeffel be kept full, the jet C will spout from the pipe D, to the horizontal diftance N M, which is double the length of the perpendicular Dd. If two other pipes, C and E, be alfo fixed in the fide

of

of the veffel, at equal diftances above and below the pipe D, the perpendicular Cc and Ee, from these pipes to the femicircle will be equal, and the jets F and H, which spout from them, will each go to the fame horizontal distance NK; which is double the length of either the equal perpendiculars Cc and Ee.

CHAP. XIX.

HYDRAULICS.

SECT. I.

OF PUMP-WORK.

HYDRAULICS is that part of the doctrine of fluids which treats of the properties of fluids in motion, with a special attention to artificial water-works: and in this sense it stands opposed to hydroftatics, which concern fluids, as they remain at reft.

The laws of fluid bodies, as given in the laft chapter, obtain alfo in this, and therefore need not be repeated.

The

The greatest benefit mankind has received from the science of hydraulics is the construction of the water-pump, first invented by Ctefebes, a mathematician in Alexandria, about 120 years before Chrift; and it depends for its action upon the preffure of the atmosphere.

That the preffure of the air on the furface of the water is the cause of the water rifing in the pump, has partly beendemonftrated in Pneumatics; for as the preffure of the air caufes the mercury to afcend in the tube of the exhausted barometer; fo the fame preffure upon the furface of the water in a well causes the water to ascend in the pump, but to a far greater height: for the mercury in the barometer rifes only to 29 inches at a medium; whereas the water in the tube of a pump will rife to 33 feet at a medium, which is found equal in weight to a column of mercury of the fame diameter, but of 29 inches in height; the mercury being near 14 times heavier than water.

That it is the preffure of the atmosphere which causes both the water and the mercury to ascend, has been sufficiently proved by numberless experiments; and may be shown by an exhaufting fyringe, commonly termed a fucking fyringe. Let this be fixed in a tranfparent tube, and the lower end thereof put in a jar of mercury or water, and the whole enclosed within a tall receiver; then, if the pifton of the fyringe be raised before the air is exhausted from the receiver, the mercury or water will immediately follow it; but after the air is exhaufted, if the pifton be raised, the fluid will not follow.

Therefore, what is called fuction, in hydraulic machines, is nothing more than when, by any mechanical contrivance, the preffure of the air is in any place abated, the adjacent matter being urged on by the weight of the atmosphere, will tend to that place; and if the matter be fluid, it will rise so far above its common level, till, by its abfolute weight, a just equality is made: to preserve that equilibrium which always obtains in nature.

of

Of the pump, there are fimply three kinds, viz. the fucking, the forcing, and the lifting pump. By the former, the water is raised by the general preffure of the atmosphere on the furface of the water in the well, and cannot be raised to a greater height than 33 feet, as before obferved; though, in practice, it is feldom raised above 28 feet, because the air is not always denfe enough to fupport a column of water of 33 feet.. By the two latter, water may be raised to any height, having an adequate apparatus, and fufficient power.

Of the Sucking Pump.

The fucking pump is that in most common use, and con fifts of a tube or pipe, open at each end, having within a fliding pifton, as large as the bore of the pipe, and which fits the pipe fo exactly, as to admit no air to pafs between it and the pipe. The pipe is called the barrel.

If the lower end of the barrel B be immersed in water (fig. 1, plate 24), and the pifton D be raised, a vacuum will be made in the barrel, by lifting up the column of upper air from A to D, and thereby permitting the air in the lower part of the barrel to expand itself; and the atmosphere preffing upon the furface of the water in the well, will force it to follow the pifton, and that even to the height of 33 feet, if the stroke could be of that continued length. But when the pifton is let down again in the barrel, the water will fall with it; to prevent which, there is a valve fixed in fome convenient part of the barrel, as at C, which valve confifts of a wooden frame A (fig. 2), exactly fitted to the bore of the barrel, and a leather flap B, lined with lead, in order to give it fufficient weight and strength. This valve opening with the upward motion of the water, and again clofing when the pifton is let down, serves to retain the water above, which flows through it: and 3 &

VOL. II.