6. =Removable Heel-Calks that do not Require Sharpening.=—The undeniable fact that all chisel-shaped or pyramid-shaped sharp calks become dull in time, and must then either be sharpened or replaced by new calks, renders shoeing not only costly, but injurious to the hoofs and annoying to the owner. This drawback is most pronounced in large cities, where the snow never lies long upon the streets, and the horse just sharp-shod is soon obliged to travel upon bare pavements. Attempts have been made to lessen this annoyance by the use of calks that do _not require sharpening_, and yet which will prevent slipping even after they have been used for a long time upon bare pavements. It cannot be denied that such calks have considerable value, and, except when the ground is covered with ice, many of these calks render excellent service. Just as the ordinary sharp calks are satisfactory and very durable outside of the large cities, so now for the first time a few of these recently invented sharp calks seem to be worthy of recommendation for city use. The following are the best: [Illustration: FIG. 175. Screw-calk with H-formed cross-section.] [Illustration: FIG. 176. Screw-calk with +-formed cross-section.] 1. Screw-calks and peg-calks with H-shaped cross-section (Fig. 175). 2. Screw-calks with +-shaped cross-section (Fig. 176). 3. Screw- and peg-calks with O-shaped cross-section (Fig. 176). 4. Screw- and peg-calks with S-shaped cross-section. 5. Angle-calks (Fig. 177). 6. Screw- and peg-calks with rubber foot-pad. 7. Screw-calks with Y star-shaped cross-section (Fig. 178). 8. Hollow wedge-calks (Fig. 179). 9. Perforated screw-calks (Fig. 180). [Illustration: FIG. 177. Corner calk.] [Illustration: FIG. 178. Star calk.] [Illustration: FIG. 179. Hollow calk.] [Illustration: FIG. 180. Perforated calk.] There is no doubt that the grip that these calks take upon the ground and their durability depend upon the diameter and the arrangement of their surfaces of friction. From all experiments made thus far it is shown that those calks which have narrow and comparatively few surfaces of friction are the least durable. [Illustration: FIG. 181. Universal screw-calk key with tap.] [Illustration: FIG. 182. Felber’s hand-vise to secure the hoof from twisting while changing the screw-calks.] To introduce and remove the calks we use a calk key or wrench. For the shop, the ordinary fork key (Fig. 181), the jaws of which are tempered, is recommended. It fits all calks. CHAPTER VI. HOOF NURTURE. Hoof nurture comprises all those measures which are employed to keep hoofs healthy, elastic, and serviceable. A. Care of Unshod Hoofs.