The slide rule is an instrument used primarily for the multiplication and division of numbers. The operation is done mechanicaly by progress equivalent to logarithmic addition and subtraction.
The numbers are read on logsrithmic scales, but the logarithms themselves do not appear in the progress.
In other words, the slide rule is a graphical log table of the accuracy (10inch) of 3-figure logs, - i.e to 3 significant figures irrespective of the position of the decimal point.
This accuracy is sufficient for all artiller problems such as:
I. B.P (Battery Position) Procedure- the complete problem can be worked out by one setting of the silde rule.
II. Interpolation- e.g. meteor telegrams.
III. Angle of sight problems.
IV. Position corrections and concentration tables.
V. T.O.B, VL shoots, crest clearance, etc.etc.
The only check necessary for any of these problems should be that obtained from a second slide rule.
The slide rule also provide a quick and accurate check of work carried out on the artillery board, and has the advantage that the check can be carried out concurrently with the work on the board.
For other work required 4 or 5 figures accuracy the slide rule is valuable as a check against gross error.
The slide rule consists of three parts:
(a) The body- the upper surface carries A and D scales, A at the top D at the bottom, at each end of the under surface "windows"are cut, each with a fine reading line. It is not necessary to use the window for the solution of any of the problems in this pamphlet, but it may be used as an alternative setting for some problems. The bevelled edge of the body carries a scale of inches, the other edge of the body carries a scale of inches, the other a scale of centimetres.
(b) The slide- This is reversible, and carries on one side scales. B and C, on the other scales S (sin) and T (tan), and S&T (sines and tans of small angles).
(c) The cursor- A sliding frame having a fine index line.
A and B scales are adjacent and identical, and are used in conjunction with one another. Each scale is divided into a right and left half which are in duplicate,except that the left half is numbered 1 to 10 and
the right hlaf 10 to 100. This allows the use of two powers of 10.
C and D scales are also identical and adjacent, but are set out at twice the scale of A and B. These scales, therefore, read only from 1 to 10, giving only one power of 10, but being larger are more accurate.
When the slide is reserved, the S scale is read in conjunction with the A scale and the T scale in conjunction with the D scale. The S & T scale is used with the A scale.