This is a stone grinding wheel for grain such as wheat.  It was originally used in a grist mill in Dayton in the 1800s.  It sits along the river trail in Dayton.
{"Image Make": "Canon", "Image Model": "Canon PowerShot G15", "Image XResolution": "180", "Image YResolution": "180", "Image ResolutionUnit": "Pixels/Inch", "Image DateTime": "2012:10:13 09:45:22", "Image YCbCrPositioning": "Centered", "Image ExifOffset": "172", "EXIF ExposureTime": "1/1250", "EXIF ISOSpeedRatings": "80", "EXIF ExifVersion": "0230", "EXIF DateTimeOriginal": "2012:10:12 12:30:05", "EXIF DateTimeDigitized": "2012:10:12 12:30:05", "EXIF ComponentsConfiguration": "YCbCr", "EXIF CompressedBitsPerPixel": "5", "EXIF ApertureValue": "24361/8200", "EXIF ExposureBiasValue": "-1/3", "EXIF MeteringMode": "Pattern", "EXIF Flash": "Flash did not fire, compulsory flash mode", "EXIF FocalLength": "1179/125", "EXIF FlashPixVersion": "0100", "EXIF ColorSpace": "sRGB", "EXIF DigitalZoomRatio": "1"}