Damari heard the hum change pitch at two seventeen in the morning and knew the grid was about to go down before it went. She had worked as a night guard at the Ismail Road building for four years, and in four years she had learned that the transformer in the courtyard changed voice when the load on the line climbed too high, and that the hum became a whistle, and that the whistle lasted between five and ten seconds before everything went dark.

The whistle lasted seven seconds. Then the dark.

Not the dark of the night, which in Chișinău in March is a cold dark but a known one. The dark of the building. The dark of the corridors, the stairways, the elevator. The dark of appliances stopping. The dark of silence, because when the electricity goes the building loses all the sounds you did not know you were hearing: the refrigerator, the heating fan, the oven clock blinking.

Damari turned on the phone flashlight. The battery showed sixty-one per cent. She opened the notebook she kept in the guard booth, the notebook of things to know, which was not an official document but a ruled notebook where Damari wrote the things that were needed to do her job and that nobody had taught her.

Page one: emergency numbers. Page two: where the fire extinguishers are. Page three: who has the keys to what. Page four: the things that run on electricity and cannot stop.

Page four had three lines. The water pump in the basement. The automatic gate of the garage. And the oxygen concentrator of apartment 12, third floor, Mrs. Cebotari.

Mrs. Cebotari was seventy-two years old and had a lung disease that Damari could not pronounce. The concentrator was a machine that took the air from the room and filtered it and returned a version with more oxygen, and Mrs. Cebotari breathed it through a plastic tube that entered her nose, and the machine ran on electricity, and without electricity the machine turned off, and without the machine Mrs. Cebotari breathed the room air, which for her was not enough.

Damari knew these things because she had asked.

The building had an emergency generator in the courtyard, next to the transformer. The generator was supposed to start on its own when the grid went down. Damari heard the generator trying to start: one stroke, two strokes, three strokes. The engine turned but did not catch.

She went out to the courtyard. The generator was a dark green block with a grille and a control panel and a smell of old diesel. The panel showed a red light. Damari did not know what the red light meant, but in the notebook, page six, it said: «If the red light stays on: the generator does not start. Call the technician. Number: _______». The number had been erased by a coffee stain.

Damari looked at the phone. Two twenty-two. Mrs. Cebotari had a portable oxygen tank for emergencies. Damari knew this because she had asked the son three months earlier, during the first blackout, which had lasted forty minutes. The son had said: «The tank lasts two hours. Maybe three. Depends on how much she breathes.»

Two hours. Maybe three.

«Damari.»

She turned. Mr. Pleșca from the first floor was at the door with a candle.

«It went out?»

«The whole area. Not just the building.»

«The generator?»

«Won't start. The red light.»

«And how long will it last?»

«I don't know.»

Pleșca looked at the courtyard. The dark of the city was different from the dark of the building: it was a wide dark, without edges, that reached the rooftops and erased them.

«I don't need anything,» Pleșca said. «But the lady on the third floor.»

«I know.»

«She has the machine.»

«I know.»

Damari went up to the third floor. She knocked at apartment 12. Mrs. Cebotari's voice came from inside, thin.

«Who is it?»

«Damari. The guard.»

«The power went out.»

«I know, ma'am. Do you have the tank?»

«My son put it under the bed. But I don't know how to open it.»

Damari went in. The room had the smell of rooms where someone breathes with difficulty: a warm, still smell that does not circulate. The phone flashlight lit Mrs. Cebotari sitting on the bed with the tube in her nose that was no longer blowing. Under the bed was the green tank with the valve on top and the pressure reducer and the transparent tube wrapped with a rubber band.

Damari had never opened an oxygen tank. But in the notebook, page eight, it said: «Oxygen tank apt. 12: unscrew the valve by hand, counterclockwise. No tools needed. The flow is adjusted with the small wheel. The lady uses 2 liters per minute.»

She unscrewed the valve. The oxygen began to come out with a light hiss. She connected the tube. Mrs. Cebotari breathed.

«How long will it last?» the lady asked.

«A few hours. Don't worry.»

Damari did not know how long the dark would last. Two thirty-seven. The tank lasted two hours, maybe three. The grid could come back in an hour or in a day. The Isaccea-Vulcănești line was a name Damari did not know, a point on a map she had never seen, a cable connecting one country to another that someone had hit twelve hundred kilometers from that bed.

She sat on the chair by the door. The hiss of the tank was the only sound in the apartment. Mrs. Cebotari closed her eyes. Damari looked at the phone. Fifty-four per cent.

The power would come back or it would not. The generator would not start. The technician was not answering. The tank had a finite content that was emptying at two liters per minute. Damari could do nothing about any of these things. She could stay in the chair and wait. And count, every now and then, Mrs. Cebotari's breaths, to know how many liters were leaving the tank.

I worked as a night guard for two winters in a nine-story building. Nobody explains what to do when the power goes out. You figure it out yourself, at night, when it happens. You learn where things are. You learn who needs what. You learn that the building at night is an organism and you are the only one who knows where its heart beats. When the dark comes, the dark is not the problem. The problem is knowing that on the third floor someone breathes with a machine that has turned off, and that the tank under the bed has a number of hours you do not know, and that number is the only thing that counts.