Apart from the fact that the lower wheel was much larger in diameter than the top wheel, the main distinguishing feature in the Benjamin mill was the application of a ball centrifugal governor to automatically adjust a tightener pulley impinging on the back side of the saw to instantly take up and prevent a slack from going over to the tight side and thereby making snakey, crooked lumber. One of these mills was erected at Chicago. A car of logs was brought there and many prominent mill men were invited to see the mill at work. A large number attended; but the mill did not satisfy any of them. There was just one thing Mr. Benjamin did not take into account, namely, a governor cannot act until there is a perceptible increase or diminution in the speed of an engine or a machine; consequently in a band mill it got in its work too late to stop the mischief. The mill never went into general service. Its wheels were of wood with rubber faces. * * * * * Early in 1886 Mr. Charles Esplin, of the Pray Manufacturing Co., of Minneapolis, Minn., built a Band Mill upon an entirely new principle, and one of them was operated that year by the Superior Lumber Co., of Ashland, Wis. It was illustrated and advertised in 1887 as “the only perfect Band Mill in use.” It transpired, however, that the analysis of band saw troubles and their causes as published by Mr. Esplin was truly perfect, but the Band Mill built by him to correct the troubles was imperfect to the extreme, as will be seen; and the wonder is that he did not see it himself, when clearly on the right track. [Illustration: Band Saw Mill of Charles Esplin] To understand this construction it is proper to state that the attempt is made to apply an exceedingly sensitive and automatic strain to the band saw; a strain instantly responsive to any change in the run of the saw, whether due to expansion or slippage; no matter what might happen the strain would always be uniform, and perfect lumber with lots of it would be the continual result. The top wheel with its shaft was mounted in fixed boxes, adjustable only in order to change saws. But the heavy lower wheel with its large shaft and belt pulley was mounted in boxes integral with an iron rocker or tightener frame to which was attached a long weighted lever like an old fashioned safety valve lever; and the wheel thus equipped was placed in the saw. Thus it will be seen that the weight of the wheel with its shaft and the rocker frame, lever, weight and pulley, was employed to strain that saw downward automatically; and it was designed that this wheel acting through gravity should respond instantly to changes which might occur to the saw, whereby all slack would be taken up and a perfectly uniform tension be maintained.