Ten Ripples a Second

A five-second exposure turned the curls into grey. Above the obstacle, the smoke still formed separate threads. Behind it, the changing swirls overlapped until their particular shapes disappeared.

Étienne-Jules Marey described the result in 1901, while explaining a machine he had built to study moving air. The longer exposure gave him something like an average appearance of the disturbance. A brief magnesium flash preserved another kind of information: the curls at one instant. The photographer could choose between them.

Marey was a French physiologist, then in his seventies, who had spent much of his career finding ways to record movements that escaped ordinary observation. His new subject was the air itself. Balloons and aeroplanes were occupying experimenters, and his earlier photographs of birds could reveal how a wing moved without showing what happened in the invisible medium supporting it.

The solution required a controlled current, something visible to carry its movement, and enough light to photograph it. Once Marey had obtained a clear picture, he still needed to know how fast the air was travelling. The stripes alone could not tell him that.

Giving the camera something to see

Marey's instruments had already made bodily movement leave a record. A pulse or the successive contacts of feet on the ground could be transmitted to a recording lever, which drew its trace on a moving surface. Photography extended that work. A timed shutter could register successive positions on a single plate, allowing distances and intervals to be examined afterwards.

For water, he made small silvered beads.

Wax was lighter than water; resin was denser. Mixing them allowed Marey to mould tiny balls that sank only slowly. He silvered their surfaces so that they caught the light, then added salt water until they remained suspended. The camera could follow those bright markers where transparent water gave it little to record.

A small submerged propeller set up a current in his water tank. An inclined sheet of glass interrupted it, and the beads recorded successive positions as they travelled around the obstacle. In one experiment described in his book Movement, the record covered three seconds at forty-two images a second. A ruler photographed with the experiment supplied the scale.

The air experiments needed a different marker. Smoke could be drawn along with the air and made visible against black. A fixed surface in a moving current also made the subject easier to examine: Marey could keep the model in the camera's field while the medium passed it.

He began the smoke work in 1899 with the help of a mechanic, Miltiade Kossonis. The equipment changed as the experiments proceeded. A narrow early passage interfered with the airflow when a substantial obstacle was inserted. Enlarging and rearranging the apparatus was part of obtaining a result worth interpreting.

Financial help came from across the Atlantic. Samuel Pierpont Langley, secretary of the Smithsonian Institution, had discussed support with Marey in 1897. The proposed grant was conditional on research into atmospheric air. It arrived around the end of 1900 and helped finance the improved machine used in 1901.

First straighten the air

In the arrangement Marey described that September, the air passed through a box with a square cross-section, half a metre on each side. The back was blackened. Glass formed the front, and a pipe below led to an electric fan that drew air down through the chamber.

Suction alone did not produce an orderly current. A little smoke released at the top wandered and whirled before any model had been introduced. Such a current would make it difficult to distinguish a disturbance caused by the test surface from one already present in the machine.

Fine silk gauze steadied the incoming flow.

Marey stretched it across a wooden frame at the top of the box. A second gauze screen below the glass window screened off disturbances from the fan. He tested the arrangement by releasing smoke again. This time it descended as a white column without wandering sideways.

The visible threads came from a furnace burning tinder. Its smoke rose through a wooden conduit, turned into a shallow distribution space and passed out through a row of small, evenly spaced tubes. The fan drew the threads into the box together. Marey compared their appearance to taut piano strings.

Successive versions used different numbers and arrangements of outlets. The densely ruled pictures from the final machine are identified with fifty-seven channels; in his popular description, Marey simply called it a row of about sixty tubes. The purpose was to mark many neighbouring parts of the same current, so that an obstacle's effects could be followed across its width.

A thin blade of micaMica×A mineral that splits readily into very thin, sometimes transparent sheets.Merriam-Webster · mica ↗ supplied an inclined test surface. The threads approaching it bent away, crowded through the passages beside it and left a confused wake behind. Other experiments used curved surfaces, triangular prisms and several planes placed near one another.

A magnesium flash made the pale smoke bright enough to record against the black background. Its own residue created another practical problem. Marey placed the flash box in the fan's outgoing passage, carrying the combustion cloud outside rather than allowing it to spoil the clear air of the room. The smoke inside the chamber was useful; a haze between the chamber and the camera was not.

Putting time into the picture

The first clear images supplied the shape of the disturbance. Marey explicitly said they did not give the air's speed at every point. A line bent around a surface whether the marked air passed it quickly or slowly.

He introduced a time signal at the source of the smoke. A small sideways movement of the tube rack put a corresponding bend into every thread. As the smoke travelled down, it carried that disturbance with it. An electric trembler repeated the movement ten times a second, sending a sequence of waves into the current.

The distance between successive waves now represented the distance travelled in one-tenth of a second. Marey placed a bright rod twenty centimetres long in the same plane as the smoke, so that it appeared in the photograph as a ruler. He could compare the wave spacing with this known length and estimate speed.

The camera did not need to take ten separate pictures. A single exposure could contain repeated marks made at known intervals. Where they bunched together near the obstacle, the marked flow was slower; where they spread farther apart, it was faster.

Exposure time determined what could still be read. The magnesium flash retained a fleeting configuration of the wake. With five seconds of illumination, the shifting eddies wrote over one another. Marey valued the longer record because it supplied an average appearance, rather than treating every trace of blur as a failure.

Both records depended on smoke following the surrounding air closely enough to serve as a marker. The photograph registered the illuminated particles. It supplied neither a direct picture of individual air molecules nor a weighing of the forces acting on a full-sized wing. The chamber, the model and the timing system defined the experiment whose result lay on the glass plate.

A closer look · 1:38 · French

Reading the smoke threads

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Turning the obstacle round

Changing a shape or its orientation changed the pattern. Marey tested a body with one rounded end and one tapered end, then reversed it. With the broad end facing the incoming flow, the threads came together again relatively quickly behind the taper. Reversed, the same general form left eddies behind its rounded end. His water experiments had already suggested the value of that comparison.

The pictures offered comparisons that could be inspected after the current had stopped. They did not yet amount to a complete aerodynamic theory. In his account Marey invited people concerned with aviation, propulsion in fluids and ventilation to use the method. He had explained it in enough detail for further experiments to follow.

The work continued around the apparatus. Pierre Noguès, one of Marey's followers, used the fifty-seven-channel machine until at least 1904, the year Marey died. Surviving negatives preserve experiments with several neighbouring inclined planes as well as single obstacles. They record an investigation extended across a series, rather than one successful picture.

Smoke for a whole day

Marey's original smoke machines have disappeared. When the Cinémathèque française reconstructed one around the turn of the twenty-first century, Laurent Albouy helped make it work again for an exhibition. Visitors would be able to move the obstacle themselves.

The reconstruction had to run all day.

Continuous public use imposed a different requirement from a laboratory session. The builders wanted the scale and vibrating time signal as well as the visible flow, but burning tinder was no longer the practical source of smoke. They used a theatrical smoke fluid instead.