In a stamp mill (Fig. 20) the foundations are usually made of hard-wood logs about 5 to 6 feet long, set on end, the bottom end resting on rock and set round with cement concrete. These are bolted together, and the “box” or mortar is bolted to them. The horizontal logs to carry the “horses” or supports for the battery frame should also be of good size, and solidly and securely bolted. The same applies to your engine bed, but whether it be of timber, or mason work, above all things provide that the whole of your work is set out square and true to save after wear and friction. Fig. 21 represents a 10-head stamp mill. My experience has been that the most effective weight for stamps and height for drop largely depends on the nature of the rock foundation. I have usually found that with medium stamps, say 7½ to 8 cwt. with fair drop and lively action, about 80 falls per minute, the best results were obtained, but the tendency of modern mill men is towards the heavier stamps, 10 cwt. and even heavier. Great improvements have been made in stamp mills since the sixteenth century, as is evident by comparing Fig. 21A with Figs. 20 and 21; even in the writer’s time they have been considerable. To find the horse power required to drive a battery, multiply the weight of one stamp by the number of stamps in the battery; the height of lift in feet by the number of lifts per minute; add one-third of the product for friction, and the result will be the number of feet-lbs. per minute; divide this by 33,000 which is the number of feet-lbs. per minute equal to 1 h.-p. and the result will be the h.-p. required. Thus if a stamp weighs 800 lbs. and you have five in the box, and each stamp has a lift of 9 in. = 0·75 ft. and strikes 80 blows per minute, then ÷ 800 × 5 × 0·75 × 80 = 240,000; one third of 240,000 = 80,000, which added to 240,000 = 320,000; and 320,000 divided by 33,000 = 9·7 h.-p. or 1·9 h.-p. each stamp. [Illustration: FIG. 20. STAMP MILL, SHOWING PART OF FOUNDATIONS.] [Illustration: FIG. 21. 10-HEAD STAMP MILL.] The total weight of a battery, including stamper box, stampers, &c, may be roughly estimated at about 1 ton per stamp. Medium weight stampers, including shank, cam, disc, head, and shoe, weigh from 600-700 lbs., and need about 3/4 h.-p. to work them. [Illustration: FIG. 21A. SIXTEENTH CENTURY STAMP MILL.]