Inflating a Dog (The Personal History, Adventures, Experiences & Observations of Peter Leroy) Eric Kraft (beautiful books to read .TXT) 📖

Chapter 37

The Mysteries of the Jet Pump Revealed

GATHER ROUND, Saturday-morning documentary viewers. Move right up close to the television screen. You can bring your bowl of cereal with you. Today we’re going to learn about fluid dynamics. We’ll begin by conducting a little experiment. Ready? Okay, here we go.

First, you’re going to have to find a small piece of paper, the sort you might use for taking a telephone message. Go on, get that piece of paper, and hurry right back.

Now hold that piece of paper in front of your mouth, grasping it only by the edge nearest your mouth, leaving the far edge free to flap. Note that the piece of paper curves downward toward the far edge, bending under its own weight.

Blow briskly and steadily across the top of the piece of paper. It rises, straightening until it is nearly horizontal.

Why does that happen? Well, by blowing across the top of the piece of paper, you create a stream of air that is moving very rapidly relative to the air below the piece of paper. Because the pressure within a moving fluid decreases as the speed of the fluid increases, the pressure above the paper becomes less than the pressure below the paper, and the greater pressure lifts the paper, pushes it up.

The statement that the pressure within a moving fluid decreases as its speed increases is known as Bernoulli’s principle, because it was formulated in 1738 by the Swiss mathematician and physicist Daniel Bernoulli, who showed that the total energy in a steadily flowing fluid system is a constant along the flow path. Because the total energy is constant, an increase in the fluid’s speed must be matched by a decrease in its pressure.

Bernoulli’s principle explains the working of an airplane’s wing. The shape of an airfoil creates an upper surface that is longer than the lower surface. Because the entire wing moves through the air as a unit, air flowing over the longer upper surface has to flow faster to get from the leading to the trailing edge, with the result that the pressure is reduced on the upper surface, making it less than the pressure on the bottom surface. The resulting pressure difference sustains the plane in flight. We call this phenomenon lift; that is to say that the conventional name for the phenomenon is lift, but I contend that we might as well call it suck. From one point of view, the wing is lifted, pushed upward, by the greater pressure below it — a kind of blowing — but from another it is sucked, pulled upward by the lower pressure above it. I submit that this is an equally valid explanation, but perhaps you disagree. Perhaps you feel that in claiming an equivalent between blowing and sucking I am just “playing with words.” If you feel that way, I invite you to perform another experiment. Assemble the apparatus shown below:

Ready? Okay. Now blow into the tube that leads to the balloon. The balloon will inflate, of course. As it inflates, hold a finger very near the end of the other tube, and you will feel that air is being driven from the flask as the inflating balloon occupies more of the interior volume. Stop blowing, and allow the balloon to deflate. Now suck on the other tube, the one that leads to the interior of the flask, not to the balloon. As you suck, the balloon will inflate. So, you see, under the right conditions, sucking is an inflationary force, and so the Night Bailer was quite right to characterize his nocturnal mission as inflation rather than evacuation.

Enter G. B. Venturi, Italian physicist, who discovered that the speed of a fluid flowing through a tube could be accelerated by introducing a tapering constriction into the flow path. Bernoulli’s principle tells us that Venturi’s constriction will also lower the fluid pressure, since an increase in velocity must lead to a decrease in pressure, and so we come at last to the jet pump.

A cross-section of a typical jet pump is shown below. Water from an outside source (the “drive water”) enters inlet section A under pressure. As the water passes from the inlet to the throat section B, its velocity increases and its pressure decreases. Then, depending on your point of view, either the relatively lower pressure within the throat section sucks or the relatively greater pressure outside the pump blows water from the bilge into the extractor inlet C. Finally, the drive water and the bilgewater are ejected into a discharge hose and carried away.

The claim is often made for jet pumps that they have no moving parts, therefore nothing to maintain, and nothing to break, but that isn’t so. The drive water must come from an outside source, under pressure, which means that the pump must be incorporated in a system that provides the water and the pressure, the medium and the motive force, and that system must having moving parts, must be subject to wear, must, inevitably, eventually, break. Everything does. The mind. The heart. The bowels. Everything.

Chapter 38

A Brief Aside on Hope

OF ALL THE LIFTING GASES, the lightest and loftiest is hope, the best of gases and the worst of gases. It swells your soul and sends you soaring. It gets you up there, where the air is rarefied, but then, when you need it most, when you are suspended so high above the daily difficulties that we call “it all,” hope has the most annoying way of leaving you flat. You cannot hold it in forever. Your lungs cry for a fresh breath, even though you know that what surrounds you is not the lifting gas of hope but the ambient air of quotidian care and disappointment, a gas with no lift at all. Unable to sustain yourself any longer, you let the last of hope escape in a sigh, and, sighing, sink.

What makes hope such an intense pleasure