To fulfil the second condition named, the engine should be at the centre of the main range of buildings, with perhaps the grinding machinery on one side, and the leather-finishing on the other; but this would be rather contrary to the third requirement. A very good plan would be to have the engine-house in the centre as suggested, but separated from the buildings on each side by brick gables; and with the boiler-house behind it, and under a separate roof, say of corrugated iron. Figs. 61, 62, from Eitner's book on American Tanning, show the arrangement of a sole-leather tannery in the United States. If it be impossible to have the engine near its work, it is in most cases better to employ a separate high-pressure engine, which may be within a glass partition, and will work all day with scarcely any attention. The loss of power in carrying steam for moderate distances through sufficiently large and well-clothed pipes is much smaller than that of long lines of shafting. The writer has known cases where fully half the indicated power of the engine was consumed in friction of the engine, shafting, and belts. High-pressure engines are as a rule to be preferred to condensing for tannery use, since the waste steam can generally be employed for heating, and both the first cost and that of maintenance are smaller. Where much fuel is used, it is quite worth while to have the cylinders indicated occasionally, both running light, and driving the machinery; much information is gained in this way as to the power spent on the various machines, and very frequently large economy is effected by proper adjustment of the valves. To work economically, an engine should be of ample power for all it has to do; and adjusted to its work, not by lowering the pressure of steam, or by checking it at the throttle-valve, but by setting the slide-valves to cut off as early in the stroke as may be. As to how early this is possible, an indicator-diagram will at once give information. In arranging shafting, moderate speeds, say 100-150 rev. per min., should be chosen for main lines, and when higher speeds are necessary, they should be got up by light and well-balanced counter-shafts, with wrought-iron pulleys. In calculating speeds, it must be remembered that they vary inversely as the size of the pulleys. Thus a 3 ft. pulley running at 100 rev., will drive a 2 ft. one at 150 rev., and a 12 in. one at 300. Of course the higher its speed, the more power any shaft will transmit, but increased friction and wear and tear soon limit this advantage. The velocity of a belt in feet per min. is obtained by multiplying the number of revolutions per minute by the girth of the pulley in feet or by its diameter multiplied by 3-1/7, or more accurately, 3·1416. [Illustration: Fig. 61.] [Illustration: Fig. 62.]