The book was his father’s, and the first eleven pages of it were water. Every night since before Wiremu could reach the dip stick, somebody had written down how deep the water stood in the tank on the ridge. That was all of it, a number and a stroke of the pen. 2010. 1940. 2010. The ninth of February said 1940, and Wiremu knew that number better than the shape of the ridge.

Anaru had kept it for nineteen years with a carpenter’s pencil. Four years ago a tractor wheel got the gate and took the left half of his grip, so he does not go up the ladder any more. He reads the book at the kitchen table and says the wind has gone quiet, and Wiremu writes that down, because that is what they do at four o’clock. Under the water numbers, starting on page twelve, Wiremu had ruled a second part of the book himself. Two columns. Left column, strokes. Right column, minutes.

The strokes are the easiest thing in the world to count if you know where to stand, which is inside the woolshed doorway out of the sun, where you can see the whole length of the pump rod through the window and count the knocks where it reaches the end of its travel. One knock is one stroke, and the crank on the mill shaft has one throw, so one stroke is one revolution of the mill, and the mill is doing the measuring.

One stroke moves three litres. The barrel is two inches across and the rod travels a metre and a half, and three litres is what the oil company man measured with a bucket in the year the mill was last rebuilt. He wrote it on the shed in white paint where it can still be read.

One hundred and eighty cows drink about fifty-five litres a day between them and there is no other water on that farm worth the name, so the herd wants nine thousand nine hundred litres a day. At three litres a stroke that is three thousand three hundred strokes. The mill turns about thirty times a minute when the wind is right, so the whole arithmetic of the farm fits inside a hundred and ten minutes. A hundred and ten minutes. Wiremu had it in his head the way other boys have the bus times. The tank is three point six metres across and two point four deep, so it holds about twenty-four thousand litres, and the dip stick is painted in white bands every hundred millimetres from the bottom.

It had not rained in the Manukau since the twenty-eighth of January. On the ninth of February the tank read 1940. On the tenth, 1910. On the eleventh, 1900, which is under the band Anaru had painted at two metres, the one that means stop. “We need a water truck,” said Tame from the road, in the voice he kept for bad arithmetic. “We need a truck on Thursday,” said Wiremu. “We need it today.” Wiremu did not argue. He went and got the book.

The ninth: sixty-one minutes. The tenth: sixty-three. The eleventh: forty-seven, and then it shut down and did not start again until nearly dark. Add those up and the mill ran a hundred and seventy-one minutes over three days, which is rather more than fifteen thousand litres up that pipe. The tank had lost eight hundred. He put the book on the kitchen table with the two columns showing. “He says the wind has gone quiet,” Wiremu said. “A hundred and seventy-one minutes is not quiet. It is not the wind.” “It is the mill,” Anaru said, which was what Wiremu wanted and not what he meant either.

Before he said anything else he had to know whether the pump was losing water, because a worn barrel and a dead wind look exactly the same from the shed doorway, and if it was the pump then the wind was not his problem at all. So on the twelfth he took the hose off the delivery pipe at the trough, put a twenty litre bucket under it and counted the strokes. Seven strokes to the bucket, and a bit over, which is three litres a stroke, the same as the paint on the wall. Every stroke came up hard against his hand on the pipe, and he did it three times. The pump was fine. It had been fine the whole fortnight.

Tame Te Koha came up the track on the Sunday with an anemometer he had bought in Auckland in 1981 and never sold. Four aluminium cups on a little spindle, and a counter on the box that clicked over every sixty metres of wind. It read twenty-two knots at two o’clock, and the mill was turning. “There you are,” said Tame. “Your father’s right. It’s a still day and the mill is lagging.” “It’s not a still day,” Wiremu said. “The vane is out.” “That is the little wheel at the back.” “That is the little wheel at the back,” Wiremu agreed, “and it is not doing its job.” Tame laughed at that and went home, because it was a boy talking about a wheel, and Wiremu went up the ladder with the book and a stopwatch.

He had watched that tail since he was tall enough to see over the fence, and it had never once occurred to him that it was doing a job. It is the thing on the end of the boom, the sheet metal vane the size of a door, and behind it there is a small four-bladed fan the width of a dinner plate that spins in the air off the back of the vane. It is called a fantail. Its business is to find where the wind is and turn the mill so the wind comes square onto the blades, the way a person turns an umbrella.

He stood underneath it for fifty minutes and it never once stopped moving. It swung right through the wind, hit the stop with a sound like a plank falling, swung back, hit the other stop, went again. Every time it reached the end of its swing the mill kept going without it for a second or two, and the vane sat leaning over past the point where the blades face the wind square.

And the noise changed. With the vane square the blades talk in one steady note, a hum you can feel through the shed floor. Off the square the note drops and grits, and inside it there is a flat knock every few seconds, and the knock was the rod hitting the end of its travel harder than it should. He counted. Two hundred and forty seconds out of the three hundred he had stood there, the vane was sitting against a stop or swinging across the back of the wind. He wrote it up in the book that night in a third column he had ruled himself, and the number was forty-one minutes out of fifty.

The trouble with a still fortnight is that it does not stay still. On the fifteenth the weather came round. The wind had been westerly for nine days and then at half past two it came forty degrees south in twenty minutes, and after that it kept going, west to south-west to south, with the cold coming down behind it. Wiremu was in the woolshed doorway with the book when the mill started to sound wrong. It had been running at thirty. It dropped to twenty-two, then to nineteen, and the knocking got louder and faster, and at four o’clock he heard the knock change into a clatter and keep going.

He went out and looked up. The vane was hard against the stop and the mill was turning with its whole face cocked away from the south, working at the edge of nothing. It ran like that for most of an hour, which is not the same as running. A hundred and forty minutes on the sixteenth, thirty-nine of them with the blades off the square, and the tank went up four hundred litres.

The fantail was worn out and that was all there was to know. Up on the mill platform you cannot stand up without holding on to something. Anaru came up the ladder behind him, one-handed, which he hated and did anyway. The fantail sits on a spindle in a bracket and is held to the tail by a weight and a spring, so it turns at a steady walking pace whichever way the wind goes. You want it fast enough to keep up with a wind that is swinging, and slow enough that it does not hunt and slam the stops.

Wiremu turned the spindle with his fingers. It went round, but it went round with a dry scraping noise, and the bracket at the top had worn into a long oval so the fan leaned as it turned. “Look at that hole,” he said. “That hole has always been that shape.” “It has not.” He showed him, and Anaru looked at it a long time.

He took out the split pin and worked the spindle free, and Anaru sponged the bracket clean with a rag and kerosene, and Wiremu found the spring stretched and rusty where it had been hooked over its pin for more years than either of them could count. He took a turn off it with the fence strainers and hooked it back one link shorter, and Anaru greased it, and the fan spun. Then they cut the gorse.

There had been gorse round the mill for as long as anyone could remember, low and thick and green the year round, on the weather side inside six metres. The wind came over the hill and hit that and broke into every kind of gust at once, and the fantail could not tell a gust from a change. It took them the whole of Sunday, and Anaru kept saying they would finish it next year.

On the twenty-ninth the wind came back in off the Manukau, west and steady, and it did not move all day. Wiremu stood in the doorway and counted. Thirty-four turns a minute. The hum was one note and the rod knocked twice in ten minutes and the vane sat square, and it touched a stop only when a gust came over the ridge. Two hours and eleven minutes. Four and a half thousand strokes, which is thirteen thousand five hundred litres through a pipe that has not carried that in three weeks, and by six the dip stick was up on the band marked two metres.

Anaru came up and stood beside him and looked at the stick standing in the water with its white bands going down into the dark. “You have written two thousand and fifty,” he said. “Yes.” “How do you know that is twenty thousand eight hundred and seventy litres?” Wiremu had been waiting two weeks for somebody to ask him that.

“The card came with the tank,” he said. “It is folded in the shed down the middle, and it says eleven hundred and fifty litres for every hundred millimetres. Twenty and a half bands is twenty-three thousand five hundred and seventy-five. Mine is twenty thousand eight hundred and seventy, and that comes out of the tank being three point six across. At the top the card is nearly four thousand litres too big.” “You have had that book eleven years.” “Yes.” “You are saying the water has been lower than you thought all this time.”

“I am saying the card is too big, and that I could get this wrong by a good deal, and so could the card.” He put his thumb on the last figure. “I want a dip stick measured with a tape from the top, and I want somebody to write down what it is in litres, and then I can stop guessing.” Anaru took a while. “All right,” he said. “But we are not doing it in the dark.”

They went down the ladder with the last number in the book wrong by two thousand seven hundred litres and no way on that farm to find out which of them was right, and below them the trough was running over into the drain, which it had not done for three weeks.