The chief requirements of a good screw-calk are, further, a =clean, deep=, but not too coarse =thread=, and but _one size of thread and tap for all calks_, so that every calk will fit in every shoe. A calk whose tap measures one half-inch (12.7 millimetres) (Whitworth) in diameter is sufficient for the heaviest shoes. The tap which is used to cut the thread in the holes for the screw-calks must be about ¹/₁₂₅ of an inch thicker than the head of the calk. In the German army the calks have a tap fifteen thirty-seconds of an inch in diameter. The coachman should be given four calks (sharp and blunt) for each shoe, and a small screw-calk key for placing and removing them. Screw toe-calks are also used, yet they require special security to prevent their becoming loose. Experimentation with the screw toe-calks, though not yet entirely satisfactory, cannot be said to have ended. The advantages of shoes provided with good screw heel-calks are so manifold that they deserve marked preference over shoes sharpened by the ordinary methods. The common objections urged against screw-calks,—namely, that they loosen and are lost, or break off, are not worthy of serious consideration, since these evils are merely the result of unskilful workmanship and poor material. _Shoes with screw heel-calks are the best shoes for winter, especially for horses that have to work hard and continuously._ [Illustration: FIG. 163. Sharp screw-calks with Whitworth thread (half-inch, natural size).] [Illustration: FIG. 164. Whitworth tap (half-inch, half natural size).] Balling with snow is prevented by using shoes narrow in the web and concave upon the ground-surface (convex iron), and thoroughly oiling the sole and frog. Sole-pads of felt, leather, or straw serve the same purpose. Balling with snow is _best_ prevented by a rubber sole-and-frog pad, or by a “stopping” of a patent hoof cement known in Germany as “huflederkitt.” 4. =Shoeing with Peg-Calks.=—The calks are merely stuck into the calk-holes, hence their name. Round and square peg-calks are used, but the former are better than the latter. The inventor of round peg-calks is Judson, an American. The shoes differ in no respect from the ordinary flat shoes. It is necessary that the tap of the calk have a moderately conical form, and exactly fit into the calk-hole of the shoe. The taper of the calk-tap is correct if for every ten thirty-seconds of an inch in length it increases or diminishes one-thirty-second of an inch in diameter (equal to one inch in every ten inches of length). [Illustration: FIG. 165. Sharp peg-calk (cog): _a_, the tap; _b_, the head.] [Illustration: FIG. 166. Blunt peg-calk: _a_, the tap; _b_, the head.] [Illustration: FIG. 167. Lower part of the reamer.]