Having removed the snow the field is carefully inspected, and quality and thickness of the ice ascertained in all parts. The field is bored with the ice auger, and the measure indicates the thickness. The auger is withdrawn and the appearance of the ice noted before the auger reaches the bottom and water fills the hole. If the snow ice is too heavy it must be removed. [Illustration: FIG. 8. FIELD PLANER.] Two methods for doing this are available. It can be cut loose and broken into chips with the snow ice planer on the field, the chips being afterwards cleared with scrapers and scoops; or it can be left on the ice and cut away by the elevator ice planer as the ice cakes pass up the endless chain ice elevator incline, on their way into the ice house. The first method is in use wherever the endless chain ice elevator is not employed. When it is to be removed by the field planer, the ice field is first laid out and plowed to the depth of the snow or sap ice to be removed. The snow ice planer follows the plowed grooves, cutting off the refuse ice. THICKNESS OF ICE.--Generally speaking, the ice is desired at an average thickness of fourteen inches; this being convenient for subsequent handling. In practice, the thickness of the ice, as it is stored or shipped from the water, varies with the exigencies of the season and the average thickness formed at the locality. In the latitude of Central and Southern Ohio six-inch ice is often stored; in the vicinity of Chicago and Omaha, ten inch; in Maine, sixteen inch, and in Minnesota, twenty inch. Ice, thirty inches thick, has been cut and stored on Lake Superior, in Northern Wisconsin, where ice forms equal in purity and brilliancy to any found in this country. The ordinary printing on the pages of a Chicago newspaper has been easily read through a block of Lake Superior ice twenty-nine inches thick. At Winnipeg, Manitoba, ice is cut forty inches in thickness. Such thick ice keeps the year through. [Illustration: FIG. 9. MARKER, WITH SWING GUIDE.] [Illustration: FIG. 10. FIELD PLOW.] [Illustration: FIG. 11. SWING GUIDE PLOWS.] LAYING OUT THE ICE FIELD.--As the ice field is inspected and mapped out for plowing, all unsound places, air holes, or shallow places where rocks or sand bars approach near to the surface, are marked for avoidance. A convenient method is to bore holes at such places, and plant pieces of brush in them. Thin ice, formed where the ice has been removed during cutting, is also marked in this manner, to give warning of the danger of venturing upon it.