The Science Notebook
Gilbert Hydraulics - Part 4

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NOTE:  This book was published in 1920, and while many of the experiments and activities here may be safely done as written, a few of them may not be considered particularly safe today.  If you try anything here, please understand that you do so at your own risk.  See our Terms of Use.

 Pages 76-100


A ship is a floating body and it displaces its own weight of water. If, for example, a ship weighs 10,000 tons it displaces 10,000 tons of water. If 5000 tons of cargo are added it floats deeper in the water and displaces 15,000 tons of water, and so on.

You will now show that a floating body displaces its own weight of water.

To illustrate the law of Archimedes for bodies which float.

Use the empty glass bottle as the floating body. Close the lower hole in the metal tank with a No. 1 stopper, put the large coupling in the upper hole, fill the tank with water until water runs out through the coupling and stops. (l)Fig. 103. Now place the bottle slowly in the water and catch the water it displaces, (2) Fig. 103.


Now make a spring balance with a bucket, (3), Fig. 103, as follows: Find a tin can around your home, punch two nail holes near the top, and attach the can to the elastic band by means of a cord, suspend the band from a nail driven in a piece of board.

Now put the bottle in the can and mark the position of the bottom of the elastic band. Then take the bottle out and pour into the can the water displaced by the bottle, (4), Fig. 103. Do you find that the displaced water weighs the same as the bottle, that is, that a floating body displaces its own weight of water?

You have here illustrated the law of Archimedes for floating bodies.

To illustrate the law of Archimedes for bodies which sink in water.

Use the bottle filled with water to represent a body which sinks in water, fill the metal tank, Fig. 104, with water until it overflows through the coupling and stops. Place the bottle in the tank slowly and catch the water it displaces.

Now attach the full bottle to the bottom of the balance (2) and mark the position of the bottom of the rubber band. Submerge the bottle in water (3), mark the position of the bottom of the rubber band again, then pour the displaced water in the can, (4). Does the balance descend to the mark (2)? That is, is the buoyant effect on the bottle equal to the weight of water displaced by the bottle?


You have here illustrated the law of Archimedes for bodies which sink in water.


To show how sunken ships are raised by means of air.

Sunken ships are raised by compressed air as illustrated in Figs. 105 and 106. Air is pumped into the ship until the ship and the air displace a weight of water slightly more than the weight of the ship; the buoyant force of the water then lifts the ship to the surface.


Illustrate this with the apparatus shown in (1), Fig. 107. Fill the bottle with water to represent the sunken ship, submerge it in a pail of water, and blow air in through the hose. Does the ship float to the surface?

Sunken ships are also raised by means of large steel pontoons filled with air as shown in Fig. 108.

Illustrate this as shown in (2), Fig. 107. Use the bottle as the sunken ship and two empty tin cans of the same size as the pontoons. Punch nail holes in the opposite sides of the top edge of each tin can, connect them as shown, force air into them a little at a time in equal amounts. Is the ship raised nearly to the surface?




Note: The ship would be floated into shallow sheltered water in this way, then repaired by divers, and floated by compressed air as described; or a coffer dam would be built around it and the water pumped out; then the repaired ship would float when the water was admitted to the dam.


The floating dry dock, Fig. 109, is a huge steel or concrete trough shaped structure with hollow sides and with large tanks along the bottom. It is open at both ends and when the tanks T.T.T., Fig. 110 are filled with water it sinks to the water line L.L. The boat then sails into the dock and is securely braced, the water is pumped out of the tanks T.T.T., the dock rises until the water line is at W.W,, and lifts the ship above water.

The dry dock lifts its own weight and the weight of the ship because it displaces a weight of water equal to the combined weights.


When the ship has been repaired or when the barnacles have been scraped from its bottom and it is ready for sea, water is again admitted to the tanks, the dock sinks to the water line LL, and the ship sails out.

You will now make an experiment to illustrate the working of a floating dry dock.

To make and operate a floating dry dock.

Use a flat cake pan to represent the dry dock, and the bottle to represent the ship.

Float the dock on water in a sink or wash basin and pour water into it until it floats with the top about 1 in. above water. This represents the real floating dry dock, with its tanks full, ready to receive the ship.

Float the bottle on the water in the dock. This represents the ship, in the dock and ready to be raised.

Now siphon the water out of the dry dock and over the edge of the sink or wash basin. This represents the water being pumped out of the tanks of a real dry dock. Do you observe that both the dock and the ship are raised as the water is siphoned out? This shows how the dock and ship are raised when the water is pumped out of the tanks of a real dry dock.

Now siphon water from the sink into the floating dry dock. Do you observe that the dock and the ship sink as water enters the dock?
This represents how the real dock sinks when water is admitted again to the ballast tanks.



To make the glass submarine submerge and rise in water.

You will observe that the glass submarine (1), Fig. 112, is hollow and that it has a hole at the stern.

Place it in a tumbler of water. Does it float?

Place it, stern down, in the bottle full to overflowing with water, close the bottle, turn it on its side, and shove the stopper in hard. Does the submarine submerge? Withdraw the stopper slightly. Does the submarine rise and also move forward suddenly?

Repeat this with the bottle between your eyes and a light and observe the air in the submarine. Is the air compressed when you shove the stopper in, and does it expand when you withdraw the stopper?

The submarine floats in the tumbler because it is lighter than an equal volume of water. It sinks in the bottle when you force the stopper in because sufficient water is forced in to make it heavier than an equal volume of water. It rises when you release the stopper because the air expands and forces sufficient water out to make it again lighter than its own volume of water.

Water is nearly incompressible but air is very compressible and when you shove the stopper in you compress the air but not the water.


Find a larger bottle and repeat these experiments.

The submarine moves forward when you withdraw the stopper because the expanding air shoots a stream of water to the rear through the stern and this drives the submarine forward.

Illustrate this with the apparatus Fig. 113. Does the stream in one direction under water force the nozzle in the other and make it writhe like a snake?


As soon as water starts to run in a pipe it rubs against the inside of the pipe and its velocity is decreased. This rubbing is called friction and it always decreases the flow of water.

To illustrate the effect of friction on running water.

Use the apparatus, (1), Fig. 114. Raise and lower the tank. Do you find that the stream from the nozzle never reaches the level of the water surface in the tank?


It does not do so because the friction in the tubes and nozzle decrease its velocity.

Use the apparatus (2), Fig. 114. Is the lower stream longer than the upper, but do you find that it does not reach as high as the upper stream? It does not, because the velocity of the water in the lower tube and nozzle is greater and therefore the friction is greater. Use the apparatus, Fig. 115. Allow the water to run into the tumbler for exactly 15 seconds and observe the amount, then close the coupling above the tee, empty the water back into the tank, transfer the elbow to the end coupling, and allow the water to run into the tumbler from the end for exactly 15 seconds. Is the flow of water less from the end? It is less because the friction in the extra pipes decreases its velocity.

It is a matter of the greatest importance that friction be taken into consideration in planning the piping for any system of water supply or water power. The facts regarding it may be stated briefly as follows :

The friction of water in pipes:

(1) Is greater in long pipes than in short pipes of the same size.

(2) Is greater in rough pipes than in smooth pipes of the same size.

(3) Is greater when the water is moving rapidly than when it is moving slowly .

4) Is greater in small pipes than in large pipes of the same length.




When you have been watering the road or garden you have probably noticed that the stream is longer when you use a nozzle than when you simply let the water flow from the end of the hose. Have you noticed, however, that you put less water on the road or garden in a given time with a nozzle than without?

To show why the stream is longer with a nozzle than without.

Use the apparatus (1), Fig. 117. Is the stream short and is the pressure low? Place a nozzle in the coupling (2), Fig. 117. Is the stream long and is the pressure high?


You have shown here that the stream from a nozzle is longer than from the hose because the pressure behind it is greater.

The pressure at any point in a pipe carrying running water is proportional to: first, the height above the point of the water in the tank; and second, to the fraction of the total resistance the running water encounters beyond the point. The pressure behind the nozzle in (2) is great because the resistance the water encounters in the nozzle is great.

To show that you put less water on a road in a given time with a nozzle than without.

Use the apparatus, Fig. 118, allow the water to run from the end of the hose into the tumbler for exactly 15 seconds and observe the amount, then insert the nozzle and repeat. Is the flow less with the nozzle than without?




You might think that the velocity of water from a nozzle would be doubled when you double the height of the water in the tank above the nozzle. You will show, however, that you must make the height four times as great to double the velocity.

To show that the velocity of water is doubled when the head is made four times as great.

Use the apparatus, Fig. 119. Allow the water to flow into the tumbler for 15 seconds with the head exactly one foot, observe the amount carefully, then repeat with the head exactly four feet. Is the amount doubled?

The head is the vertical distance the water surface in the tank is above the nozzle opening.

The velocity of water in a pipe varies as the square root of the head. That is, if you start with a head of 1 foot, and increase the head to 4 feet the velocity is doubled, √4 = 2; if you increase the head to 9 feet the velocity is trebbled 9 = 3, and so on.


If the pipe from your water tank to your house runs up and down hill, it may become stopped by an air lock as shown in Fig. 120. In (1) the tank is empty but water remains in the U part to the level of the bathroom faucet; above this is air. In (2) the tank is again filled and the bathroom faucet is open but the water does not run. It does not because the air in the pipe permits the 15


foot head at the tank to be balanced by the 15 foot head below the bathroom faucet. This is called an air lock.

The air lock can be destroyed by opening any faucet near the bottom of the U because these let out the water and then the air. It can be destroyed here by opening the basement faucet.

To illustrate an air lock.

Use the apparatus, Fig. 121. In (1) the tank is empty and the U is half full of water. In (2) the tank is filled but the water does not run. It is air locked because the air permits the 8 inch head in the U to balance the 8 inch head at the tank.

Open the tee. Is the air let out? Close the tee. Does the water flow, that is, is the air lock destroyed.?



Pneumatic engineering is the engineering which deals with air and other gases. You have already used two pneumatic appliances in the section on hydraulic engineering, namely, the siphon and the pump; these are pneumatic and also hydraulic appliances. You have also made some experiments to show that the atmosphere exerts pressure; you will begin your work in pneumatic engineering by making further experiments along this line.


To show that the atmosphere exerts pressure.

The Magdeburg hemispheres, (1) Fig. 122, are made of metal, are hollow, and are ground smooth around the edge so that they fit together air-tight. When the air is pumped out, through the handle on one side, they are hard to pull apart. The original hemispheres, (2) Fig. 122, were 14 inches in diameter and required eight horses on each side to pull them apart. When the air is pumped out there is nothing inside the hemispheres to exert pressure outward and the pressure of the atmosphere holds them together.

Show this with (1), Fig. 123. Pull the handle up and there is very little air inside to exert pressure outward. Pull out the end stopper. Does the atmosphere make this rather difficult?


Show it also with (2). Fill the quart sealer one third full of hot water, put on the rubber ring and the cover but do not seal, place the sealer in a saucepan of salt water, heat until the water in sealer has boiled for one or two minutes, seal and stand aside until quite cold. Unseal and try to lift the cover. Is it difficult?

The steam formed in the sealer drives out the air and when the steam condenses there is a vacuum above the water in the sealer. There is then no upward pressure under the cover and the atmospheric pressure on top makes it difficult to lift the cover.


When the plunger is raised in the tube, (1), Fig. 124, the atmospheric pressure on the outside forces the sheet of rubber in.

Illustrate this also by means of (2), Fig. 124. Suck air out of the tube and close the hose with a clip. Does the atmosphere force the rubber in? Turn the rubber in all directions. Is the pressure of the atmosphere equal in all directions.


A most striking method of showing that the atmosphere exerts pressure is shown in Fig. 125. A little water is placed in an empty syrup can and boiled until the steam comes out for one or two minuts. (Sic.) The can is then closed air tight and inverted in a dish of cold water. In a short time the can suddenly collapses.

The reason for this is as follows: when the steam has driven out the air there is nothing left in the can but water and steam, and when


the steam condenses in the closed can, there is nothing in the space above the water, to exert pressure outward and the can must stand the whole pressure of the atmosphere. If it is not strong enough to do this, it collapses.

Beg or buy a gallon syrup can and try this experiment, it will certainly surprise you. Be sure the opening is covered with water when you invert the can in cold water because the water will help to make the opening air-tight.

You cannot make this experiment with a glass bottle because the glass is strong enough to support the atmosphere.



The pressure of the atmosphere was first measured by an Italian named Torricelli in 1643, with apparatus similar to that shown in Fig. 126. His experiment was essentially as follows: A glass tube, 3 feet long and closed at one end, was completely filled with mercury (quicksilver) to expel the air; the open end, closed with the finger, was then inverted over a dish of mercury, and the finger was removed under mercury.

He found that some of the mercury came out of the tube but that a column remained to a height of about 30 inches above the surface of the mercury in the dish.

Since no air enters the tube, the space above the mercury in the tube has nothing in it, that is, it is a vacuum. There is, therefore, no pressure downward on the surface of the mercury in the tube, and the pressure of the atmosphere downward on the surface of the mercury in the dish supports the column of mercury in the tube.



If this experiment is repeated with the tube shown in Fig. 127, the top of the mercury in the long closed tube is 30 inches above the top of the mercury in the short open tube. Since, as you will show shortly, this height is independent of the area of cross section of the tube, we can consider this to be just 1 square inch.

The pressure of the atmosphere on 1 square inch at A, then, supports a column of mercury BC which is 1 square inch in area and 30 inches high, that is, it supports 30 cubic inches of mercury.

Now 1 cubic inch of mercury weighs .49 lbs. (nearly 1/2 lb.) and 30 cubic inches of mercury weigh .49 x 30 = 14.7 lbs. The pressure of the atmosphere is therefore 14.7 lbs. per square inch, (nearly 15 lbs. per square inch).

It is a very astonishing fact that the atmosphere exerts 14.7 lbs. pressure on each square inch of every thing at the surface of the earth. It is at first almost unbelievable, but you have already made experiments which illustrate this pressure and you will make others as you proceed.

To measure the pressure of the atmosphere.

If you have a spring balance you can measure the pressure of the atmosphere directly with the apparatus, Fig. 128, as follows.

The diameter of the plunger is a little over 5/8 inches and therefore its area is 3/10 square inch. If then the pressure of the atmosphere is 15 lbs. on 1 square inch it is 15 x  3/10 = 4 1/2 lbs. on 3/10 square inch.

Soap the plunger well to make it slippery, shove it about 3/4 way into the tube, fill the remaining 1/4 of the tube with water, and insert a solid rubber stopper in this end, (1). Now turn the tube so that the plunger handle points vertically upward, and pour a little water in above the plunger to make it air-tight, (2).


Now to measure the pressure of the atmosphere, attach the plunger handle to a spring balance, hold the tube firmly against the table, and ask your partner to pull upward on the spring balance while you observe the pull recorded on the balance, (3).

Ask him to lift the balance slowly until the plunger is about two inches above the water, then ask him to allow the balance to go back slowly until the plunger is only about 1 inch above the water. While he is doing this you must read the average pull on the balance.

Do you find this average pull to be 72 ozs. or 4 1/2 lbs?

Note: While your partner is raising the plunger, the friction of the plunger against the sides of the tube is working against the balance and the pull will be over 4 l / 2 lbs; but while he is lowering the plunger, the friction will be working with the balance and the pull will be less than 4 1/2 lbs. The average will be about 4 1/2 lbs.

You have shown here that the pressure of the atmosphere is 4 1/2 lbs. on 3/10 sq. in. or 4 1/2 x 10/3 = 15 lbs. on 1 square inch.


The barometer, Fig. 129, is the chief instrument used by the Weather Bureau in forecasting the weather. It Is an apparatus similar to that used by Torricelli in his experiment. The pressure of the atmosphere on the mercury in the open tube or cup supports a column of mercury about


30 in. high in the long closed tube. The pressure of the atmosphere varies from hour to hour and the height of the mercury column varies with it. Weather forecasts are based on this variation.

It has been found that when the mercury falls much below 30 in., because the atmospheric pressure is low, bad weather may be expected; and when the mercury rises much above 30 inches, because the atmospheric pressure is high, good weather may be expected. The extreme variations are from about 29 in. to 31 in.

The barometer (2) is the type used on ships, and when a sailor says "the glass is falling" he means that the mercury in the glass tube is sinking below 30 in. and that bad weather is to be expected; when he says "the glass is rising," he means that the mercury is rising above 30 in. and that fine weather is to be expected.

Another type of barometer is shown in Fig. 130. It is called an aneroid barometer because it contains no liquid. It has a flat, round, air tight metal box from which the air is exhausted. The atmospheric pressure would force together the top and bottom of this box if they were not kept apart by the strong spring shown


above the box. If the atmospheric pressure increases, the spring is forced down; if the pressure decreases, the spring rises. The movements are very small, but they are magnified by levers and are communicated to the pointer by means of a rack and pinion.


The air zones of a modern battle are illustrated in Fig. 131 and the altitude gauge by means of which the airmen know their height is shown in Fig. 132. This altitude gauge is a recording aneroid barometer called a barograph. It records the height of the airplane in feet and is suspended free of the airplane by four elastic straps which protect it, to some extent, from the vibration of the machine.

The construction of the barograph is as follows. It has five or six flat metal boxes, exhausted of air, similar to the box in the ordinary aneroid. These boxes are expanded by a strong spring, as the height increases, and this movement is communicated to the long pointer. On the end of the pointer there is a pen, with a supply of ink, which bears against a sheet of paper on a drum revolved by clockwork. The pen makes a continuous record on the paper of the height in feet.

Go to Gilbert Hydraulics - Part V    or   Back to the A.C. Gilbert Collection

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