The machinery of nature, whether with sand heaps or sand grains or coral pebbles, is the same. It is impelled by currents flowing over loose materials which make eddies in the lee of lumps. The eddies are either billows or cyclones. At the middle they are billows; at the ends they are cyclones. The billow eddies obstruct the heaping. The cyclone eddies lengthen the heaps right and left of the current direction. George Darwin studied the eddies by means of a drop of thick ink in a glass tank on top of a ripple ridgelet. The ink migrated to form underwater billows and cyclones, or vortexes. He used a dropper to place the ink globule, and then watched the vortexes form as he oscillated the tank. Low parts travel fastest, namely the points. High parts build on the upstream side, and travel slowest, and the stuff tumbles over the crest line and is corniced by the eddy. Snow does it, pebbles under sea do it, and marine life adapts itself to it, wherever the food supply is best. In the study of ripplemarks, Harry Gummeré, a graduate in astronomy, was my collaborator. Ripplemarks are made by back-and-forth eddies on the bottom, while big waves oscillate the water. We moved the bottom instead of making waves in the water. A glass plate sprinkled with sand under water in a tank was oscillated back and forth horizontally. It was clamped under a carriage which oscillated on wire tracks stretched across the tank. A string pulled the carriage against an elastic on the other side. A wooden wheel and crank, set upright edgeways, had holes and pegs to pull on the string, and the crank turns were timed with a metronome. The holes in the flat wheel were a centimeter apart, so that a revolution of the wheel pulled the string for every two centimeters of travel of the carriage. Thus the sand-covered plate was jerked back and forth under water two centimeters, four centimeters, six centimeters, and so forth; once a second, or two seconds, or three seconds, and so forth, by beats of the metronome. The result was beautiful ripplemarks on a glass which could be lifted out of the water, dried, and placed over blueprint paper to preserve the record. The sizes of ridge to ridge ripples were from a fraction of an inch to two or more inches. The little ones diminished to zero when the jerking was small, the big ones washed out when the jerking was too big. The blueprints showed that both length and speed of strokes (amplitude and acceleration of motion) made the ripples increase in size, and somewhere between the largest and smallest sand ripples was the optimum perfection of ripple form. The blueprints look like mackerel skies. And mackerel-sky clouds are billows of condensation between an upper cold stream of air and a lower moist one. In between are the same back-and-forth billows of vortex as in our sand.