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Farmers will soon be tapping into “cow power” to help them water their crops. Gary Henderson of Gravitational Systems in New York has developed a pump that’s driven by cows walking over it. The pump has a horizontal metal plate that can be embedded in the ground along any path the cows use regularly. When a cow steps onto the plate, it compresses a water-filled plastic bladder underneath. This pushes water out through a one-way valve into a pipe. As the beast steps off the plate, springs inside the bladder force it to expand, drawing water in through another one-way valve.…