The first mill was a hole made in a stationary rock (Fig. 1). The grain was placed in the hole and crushed with a stone held in the hand. On Centre street in Trenton, New Jersey, not many years ago one of these primitive mills could still be seen and there are evidences that such mills once existed in all parts of the world. In those places where the earth did not supply the stationary rock, stones were brought from afar and hollowed out into cup-like form and in these the grinding was done. [Illustration: FIG. 2.--THE KNOCKING-STANE.] The mill which consisted of a hole in a rock and a stone in the hands was followed by the "knocking-stane" and mallet (Fig. 2). The "knocking-stane" was a mortar, or cup-shaped vessel made of stone; the mallet was usually made of wood. The grain was placed in the mortar and struck repeatedly with the mallet, the beating being kept up until a coarse flour was produced. This is an exceedingly rude method of crushing grain, yet this is the way the people in some parts of Scotland grind their barley at the present time. [Illustration: FIG. 3.--MORTAR AND PESTLE MILL.] At a very early date the "knocking-stane" was laid aside for the mortar and pestle (Fig. 3) almost everywhere. In this mill the grain instead of being struck with a hammer was pounded with a pestle. The bottom of the pestle was frequently covered with iron in which grooves were cut. As the man pounded he found that when he gave the pestle a twirling or rotary motion as it fell it ground the grain much faster. We may be sure that after this was learned the twirling motion was always given. The mortar and pestle were followed by the slab-mill (Fig. 4). Here the grain was ground by being rubbed between two stones. Dr. Livingstone, the great African explorer, gives the following description of a slab-mill which he saw in operation in South Africa. "The operator kneeling grasps the upper millstone with both hands and works it backwards and forwards in the hollow of the lower millstone, in the same way that a baker works his dough. The weight of the person is brought to bear on the movable stone and while it is pressed and pushed forward and backward one hand supplies every now and then a little grain to be bruised and ground." [Illustration: FIG. 4.--THE SLAB-MILL.] [Illustration: FIG. 5.--THE UPPER AND NETHER MILLSTONE.]