Big whirls have little whirls that feed on their velocity,
And little whirls have lesser whirls and so on to viscosity.
—Lewis Fry Richardson 1

When my boyfriend asked me whether I was afraid of him, I told him I was not. When he told me I should be, I told him I was not. We hadn’t been dating long but I already loved him. He worked on the boat motor in the sun while I sat on the dock, memorizing his biceps and reading one of his engineering textbooks I pulled from his bookcase. “You don’t need to read that,” he said, but I wouldn’t accept that there were any books whose meaning I couldn’t break. “It’s too beautiful not to make sense to me,” I said. “Too poetic.” He said, just like the time he told me he loved me while he was sleeping and I was awake all night, “It doesn’t mean what you want it to mean.”

The science of fluid dynamics describes the motions of liquids and gases and their interactions with solid bodies. There are many ways to further subdivide fluid dynamics into special subjects. The plan of this book is to make the division into compressible and incompressible flows. Compressible flows are those in which changes in the fluid density [are]

impossible to predict: some days, love’s viscosity was high, shown to me by his trust in my hands on the wheel of his boat. Love drove my slender biceps to scrub and wax fiberglass for days while he took to the uncovered, aging motor like a surgeon on an open heart. On the water, I studied his movement as he flipped his body upside-down, every muscle a hard cord when his wakeboard hit the lake. Other days, we fought. I was mad because he wouldn’t say he loved me; he was mad because I was an idiot.

Incompressible flows, of either gases or liquids, are flows where density changes in the fluid are not an important part of the

fights we had. Whether I was having fun. Whether I was being bipolar. Whether I could help it. “We’re more alike than you think,” he said when I told him there was a thing inside me that I couldn’t control. We never fought about the boat or the water. The only safe subject, eventually, was the one I couldn’t understand.

The wing moves in a straight path, while the ship’s propeller blades are rotating. The propeller operates in water, a nearly incompressible liquid, whereas the wing operates in air, a very compressible gas. The densities of these two fluids differ by a factor of

whether you believe what went on in my head or what came from his mouth the night I woke up with my nose between his finger and thumb, my mouth under his pressed palm. The water kicked at the beam that suspended the bed above the lake. The white moon stared hard. My boyfriend said, “You were snoring,” and released me.

In spite of these obvious differences, these two flows are governed by the same laws, and their fluid dynamics are very similar. The purpose of the wing is to lift the airplane, while the purpose of the propeller is to

produce the thrust on the boat. I begged us forward, into our second winter, into seriousness as our friends put on suits and gowns and got serious adult marriages. He poured water on my cat and picked her up by her neck. In secret, I started smoking again.

The interface between a solid and a fluid is imagined to be a surface where the density jumps from one

brand of magic to the next, swelling and shrinking under the weight of my boyfriend and his board. He made the water seem like pavement. I tried the board, but the water flowed everywhere, never yielding to me as it did for him.

It is hard to give a precise description of a fundamental concept such as mass, energy, or force. They are hazy ideas. We can describe their characteristics, tell how they act, express their relation to other ideas, but when it comes to saying what they are, we must resort to

lying to ourselves. I don’t remember whether I was afraid of him. I remember the way I would scream at him, drunk on beer gone hot in the sun that anointed the lake, but I’ve forgotten the topic and remember only the subtext. I don’t even remember the words he used to convince me I should submit to being fucked in ways that hurt me. Years later, my psychiatrist told me my outbursts had probably been PTSD episodes. I didn’t remember being afraid. I remembered feeling like I was in love. I can’t make the division because I still don’t understand the terms.

After he broke up with me, I chain-smoked for years. Water is nearly incompressible. Air is compressible. I still wonder whether I really did love him right up to the moment he started to smother me, whether I loved him in the second I thought I was going to die.