112 THE SLIDE RULE. affect the result. Using the lower scales and working carefully the error should not greatly exceed 0*15 per cent, with short calcu- lations. With successive settings, the discrepancy need not necessarily be greater, as the errors may be neutralised ; but with rapid working the percentage error may be doubled. However, much depends upon the graduation of the scales. Rules in which one or more of the indices have been thickened to conceal some slight inaccuracy should be avoided. The line on the cursor should be sharp and fine and both slide and cursor should move smoothly or good work cannot be done. Occasionally a little vaseline or clean tallow should be applied to the edges of the slide and cursor. That the percentage error is constant throughout the scale is seen by setting 1 on C to 1*01 on D, when under 2 is 2 '02 ; under 3, 3*03 ; under 5, 5*05, etc., the several readings showing a uniform error of 1 per cent. A method of obtaining a closer reading of a first setting or of a result on D has been suggested to the author by Mr. M. Ainslie, B.Sc. If any graduation, as 4 on C, is set to 3 on D, it is seen that 4 main divisions on C (40-44) are equal in scale length to 3 main divisions on D (30-33). Hence, very approximately, 1 division on C is equal to 0*75 of a division on 1), this ratio being shown, of course, on D under 10 on C. Suppose v/4'3 to be required. Set- ting the cursor to 4*3 on A, it is seen that the root is something more than 2*06. Move the slide until a main division is found on C, which exactly corresponds to the interval between 2 and the cursor line, on D. The division 27-28 just fits, giving a reading under 10 on C, of 74. Hence the root is read as 2*074. For the higher parts of the scale, the subdivisions, 1-1*1, etc., are used in place of main divisions. The method is probably more interesting than useful, since in most operations the inaccuracies introduced in making settings will impose a limit on the reliable figures of the result. For the majority of engineering calculations, the slide rule will give an accuracy consistent with the accuracy of the data usually available. For some purposes, however, logarithmic section j> (the use of which the author has advocated for the last twenty years) will be found especially useful, more particularly in calcula- tions involving exponential form aloe.
