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A description of the instrument from Thacher's manual
Upon the lower lines of the bars, and in contact with the slide, are two other scales of the same length and arranged in the same manner as those on the slide, there being a complete scale both on the left and right of the centre. When the commencement of the scales on the slide and envelope are in contact all divisions on the one are opposite corresponding divisions of the other. Upon the upper lines of the bars, and not in contact with the slide, is a scale of roots. This scale is double the length of the others, and is divided into double the number of equal parts. It occupies the entire upper part of the bars, both on the left and right of the dividing centre line. These parts are laid off in regular order, and in such manner that any number on the lower line of bar is the square of the number opposite to it on the upper line, and any number on the upper line of bar is the square root of the number opposite to it on the lower line. For numbers having 1, 3, 5, etc., or an odd number of places, their roots are found on the left, and for numbers having 2, 4, 6, etc., or an even number of places, their roots are found on the right. By the rotary movement of the slide any line on it may be brought opposite to any line on the envelope, and by the longitudinal movement any graduations or subdivisions of these lines may be brought opposite to, or in contact with, each other. One-fortieth of a revolution is equivalent to nine inches of longitudinal movement.
When the slide is set in position for use the divisions on it appear of about equal length on both edges of the bars, and two lines of figures marking their values appear in each opening between the bars, two similar lines also being covered by each bar. The divisions on the upper lines are transferred to the slide by means of a pointer fitting over the bars. This pointer is also convenient for retaining the position of any division on either line while the slide is being revolved into the required position.
Near the commencement of each scale on the envelope and slide is a heavy black mark, designed to readily catch the eye during the rapid movement of the parts. The former will be found to expidite multiplication, and the latter division.
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