Where natural tanning materials are used, either entirely or in conjunction with extracts, the leaching is done in a series of large square pits, four of which would suffice for a small yard, while a very large tanning would need twelve or sixteen. Oak bark, which is usually delivered to the tannery in strips measuring 3 to 6 ft. in length, must be chopped or ground into small pieces by machine (Fig. 21). A measured quantity (a certain number of baskets or skeps full) is placed in the empty pits, which are then filled with water. The liquor is pumped from these pits, as required, to others in which the hides are tanned. The hard fruit of myrobalans, which somewhat resembles nutmegs, is powdered in a disintegrator or special crusher. [Illustration: FIG. 22 TAN-YARD ("Dri-ped" Tannery)] [Illustration: FIG. 23 HIDE ROUNDED FOR SOLE LEATHER] The three principal vegetable tannages are those for sole, dressing, and light leathers, and it is obvious that a careful selection of materials is necessary to obtain the desired effects. For this reason, oak bark, although a good tanning material for dressing leather, is insufficient to produce a firm sole leather, and, therefore, even in the so-called pure bark tannages, valonia or other suitable tannin is used towards the end of the process in order to increase the solidity and waterproof quality of the leather. There are two general methods of tanning sole leather, namely, the pit and the drum tannage. In the first case, the hides are passed through three series of pits containing gradually increased strengths of tannin (Fig. 22). These series are technically described as "suspenders" or "colouring pits," "handlers," and "layers" or "layaways." To facilitate handling and economise tanning materials, hides are "rounded" (_i.e._, cut into sections) either before tanning or after the hides have passed through the suspenders (Fig. 23). The obvious advantage of the former method is that the offal (shoulders and bellies) removed can be chrome tanned if required. The suspenders may consist of any number of pits from six to twenty-four, according to the size of the tannery. As the name implies, the hides are suspended in the liquors from poles, which extend across the top of the pit. The hides are attached to the poles by means of stout cord or copper hooks. Various mechanical appliances are in use to supersede the old method of allowing the hides to rest during this process. These methods save time and tend to produce even coloration, the latter being a very important point in the early stages of tanning. Some of the methods advocated, however, are too vigorous for the hides at this stage, when the chief object should be to preserve as much gelatine as possible. The best mechanical system is that which gently raises and lowers the hides in the liquors. Some of the American tanners use a mechanical contrivance known as the rocker, which consists of a stout wooden beam rocking from a central pivot,