Hollow calk.
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.
Fig. 181.
Universal screw-calk key with tap.
Fig. 182.
