Frank's notebook weighed three hundred and twenty grams, he had weighed it once out of curiosity on the laboratory scale, the one used for dosing the coagulant, and three hundred and twenty grams had seemed like too little for thirty years of things nobody else knew, thirty years of valve sounds and joints that give way and pumps that change tone before breaking and that way water has of changing smell when the iron in old pipes begins to dissolve, a smell the protocol calls 'metallic taste' and that Frank called 'the pipe is eating itself' because the pipe was indeed eating itself, layer by layer, the way rust eats a nail, only that you can see the nail and you cannot see the pipe, the pipe is underground, the pipe is under the road, the pipe is under the school where children drink the water the pipe carries and the pipe carries the water Frank treats and Frank treats the water with his hands and with the notebook and with thirty years of five-a.m. mornings in a plant that next year will cost the district one hundred and forty thousand dollars in maintenance the district does not have and that the district will replace with an automated system that reads the sensors and adjusts the pumps and that will work, 'oh it will work,' it will work at ninety per cent because ninety per cent is what the sensors see and the pumps adjust and the software calculates, but the ten per cent is what Frank does with his fingertips on the flange of valve 7 when the water temperature drops below four degrees.

Frank fell ill in February. A pneumonia, not serious but enough for two weeks at home, two weeks in which the plant ran without Frank because the plant had the sensors and the displays and the software and the new kid the district had sent with his forty-hour certificate and his tablet and his way of looking at the numbers as if the numbers were reality, and the numbers were reality, 'a reality,' the one the sensors produced and the displays showed and the software interpreted, but there was another reality the sensors did not produce and the displays did not show and the software did not interpret, the reality of the sound of valve 7 and the smell of iron and the vibration of the flange and the water hammer the manual does not mention, and that other reality for two weeks was read by nobody.

Frank came back on a Monday. The plant was running. The water was flowing. The displays showed numbers in range. The notebook was on the desk where Frank had left it. Nobody had opened it. Frank opened it to page one hundred and eighty, the last page written, dated February 3, the day before the pneumonia: 'V7 slight vibration, not on display, pH 7.2 (display 7.1, difference 0.1, in range but rising for three days).' Frank went to valve 7 and touched the flange. The vibration was no longer slight. It was constant. The pH on the display read 7.4. The operating range went up to 8.5. No alarm. No new kid who had noticed that the 7.1 of three weeks ago had become 7.4 and that 7.4 was still in range but that the direction mattered more than the number, 'the direction mattered more than the number,' and Frank knew it because in 2009 the pH had risen from 7.0 to 7.6 in four weeks and nobody had noticed until it reached 8.2 and the water began to taste like the pipe and two people had called the district.

Frank corrected. He opened valve 12 by a quarter turn. He checked the coagulant doser. He cleaned the pH sensor that had a limescale deposit shifting the reading by one tenth. One tenth. The tenth that separated the number on the display from the real number, the tenth that separated the world of sensors from the world of fingers, the tenth that Frank corrected every day and that for two weeks nobody had corrected and that in two weeks had become three tenths and that in a year would become a full point and that in one point there were fourteen thousand taps and fourteen thousand glasses of water and fourteen thousand people who did not know the water they drank was good because a man with a three-hundred-and-twenty-gram notebook touched a valve with his fingertips every morning at five ten.

Frank was not indispensable. The plant ran without him. The water flowed. The numbers were in range. The new kid had not called the district, had not noticed the vibration, had not opened the notebook. The system did not need Frank. The system needed someone to press the buttons and read the displays and the new kid did that. But the system did not know that the tenth Frank corrected was the tenth that kept the system from noticing itself, and a system that does not notice itself is a system that works until it doesn't, and when it stops working it stops all at once, suddenly, like a rope that breaks at its thinnest point, and the thinnest point was the point where Frank put his fingertips, the point the display could not see, the point the notebook described with the words of someone who touches and not with the numbers of someone who watches.

The notebook stayed on the desk. Frank did not take it home. He did not hide it. He left it open to page one hundred and eighty, the one from February 3, with the slight vibration and the rising pH and the difference of one tenth between the display and the world. Anyone could have read it. Nobody did.