Improvements in the Engineer's Slide-rule.
Year: 1891

page_text: Date of Application, 6th Oct., 1891-Accepted, 7th Nov., 1891
COMPLETE SPECIFICATION.
Improvements in the Engineer's Slide-rule.
I, HERBERT JOHN ALLISON, of 52 Chancery Lane, London, in the County of
Middlesex, Patent Agent, do hereby declare the nature of this invention (which
has been communicated to me from abroad by the Keuffel & Esser Company
Incorporated), of the City and State of New York, U.S.A., and in what manner
the same is to be performed, to be particularly ascertained and described by the
following statement :-
The rule or body is composed of two parallel bars of adequate depth and
Inoderate thickness, each one having a groove in its edge which is presented
inwardly towards the slide These bars are held rigidly together by cross-pieces,
attached one on each face at each end, and are so distanced from each other that
the slide which is of equal thickness with the bars, may fit nicely between them
and be held there in the groove running from end to end of the inner edges of the
bars, but only so held that it may be easily and smoothly drawn out at either end.
Each bar is graduated with the proper logarithmic scale in the usual order
progressing from left to right. There is one single scale on the lower bar running
from one to ten and from left to right, whilst on the upper bar there are two
similar scales, but each one being half the length of the lower one, these upper
scales also progressing from left to right being commonly known as the scale of
squares. These two lines of scales are graduated not only on one face of the bars
but similarly and in every respect identically on both faces.
There are in the calculation of many mathematical problems by means of the
slide-rule, cases in which it is common to reverse, or as it is technically termed, to
invert the position of the slide, which invertion is rendered practicable by making
each edge of the slide with its rebates similar, and correspondingly preparing the
edges of the recess in the body of the rule, and then operating by drawing out the
slide its entire length so as to disconnect it, turning it around end for end, and
again sliding it into the body of the rule. When such problems are interwoven
with others in which the slide is used in its usual position, the changes not only
involve labor, but render the solution of the problem liable to error, and endanger
the bending or breaking of the slide, and obviously tend to reduce the device to a
loosely fitted instrument, unfit for the nice determination of quantities to several
places of integers or decimals, of which, in its best condition, it is capable.
This invention presents on one face of the slide two accurately graduated scales
in the usual or ordinary order progressing from left to right, corresponding and
coinciding with two similar scales on one face of the rule, and on the other face of
the slide two accurately graduated scales in the reversed order, progressing from
right to left, corresponding with two scales in the usual order on the other face of
the rule (but not coinciding with them), being eight scales in all, all the indices or
terminal lines of which are made to exactly coincide with each other, so that such
problems are solved at once by inspection without inverting the slides or making
any equivalent change.
The rule is therefore double-faced, and the slide is equally double-faced, the two
combined forming a double-faced slide-rule, so constructed as to allow both faces to
be used simultaneously.
The slide, as I have said, presents on one face two lines of scales in the usual
order, progressing from left to right and corresponding in every respect and
coinciding with the lines of scales on the two bars of the rule, while on the other
face of the slide are two similar lines of scales, but in reversed order, that is,
progressing from right to left, their indices alone coinciding exactly with the

page_text: indices of the lines of scales on either fabe of the two bars. The upper scale on
each face of the slide is a double one or scale of squares, and the lower scale on
each face of the slide is a single one or scale of square roots. These scales in
reversed order are therefore equivalent to scales of the ordinary slide inverted, but
have in addition to saving of labor and the avoidance of the risk of injury to the
instrument, the advantage of being thus placed contiguous to their corresponding
scales on the upper and lower bars of the body and are easier to read as the
figures denoting the values of the graduations stand in their ordinary positions
instead of being turned upside down.
The accompanying drawings form a part of this Specification and represent what
is considered the best means of carrying out the invention.
Fig. 1 is a face view of one face, the finest graduations being omitted," but
enough being shown to indicate the character of the whole.
Fig. 2 is a view of the opposite face correspondingly presented.
Fig. 3 is a cross-section on the line - in Fig. 1.
Fig. 4 is a cross-section on the line y-y in Fig. 1.
Similar letters of reference indicate like parts in all the figures.
The slide-rule has the usual Arabic figures to mark its peculiar diminishing
graduations. Certain capital letters, A, B, C, D, have long been used to designate
the different lines of graduations on the slide and on the rule. I will not, therefore,
use any of these letters as reference marks to designate parts of the mechanical
construction, but will use therefor lower-case letters in the latter part of the
alphabet.
The upper part of the body is marked w', the lower part of the body w³½ and the
rigid connecting pieces at the ends to. The slide is marked z.
The several graduated lines on the one face of this slide rule are marked-by
their usual designations, A, B, C, and D, A and D-being on the two bars, and B-
and C on the slide. The two lines of graduations on the other face of the two bars
of the rule are similarly marked A and D; while the two lines of graduations on
the corresponding face of the slide are designated by the proper letters Band C,
with a capital appended indicating that they are inverted, or reversed.. One
face of the slide rule presents the ordinary standard four lines of graduations, A, B₁
C; D. The opposite side of the slide-rule has its upper and lower lines of
graduations identical in all respects and coinciding with the lines A and D, and
they are similarly marked, but the intermediate scales, those carried on the slide on
this side are inverted and are marked B. I. and C. I, meaning respectively B'reversed-
and C reversed, the scale B reversed being next to the scale-A, and the scale C
reversed next to the scale D of the rule.
The scales A and B are alike, short radii, the scales C and D-are alike long.
radii. The scale B. I. is short radii reversed, and the scale C: I.-is long radius
reversed.'
A runner or metallic band u encircles the whole rule and slides freely along its
whole length, enabling corresponding points on either face and on any scale to be
noted, and complex problems to be solved without the necessity of reading off any
of the intermediate results.
When in the use of my duplex slide-rule it is required to determine a problem
in which the ratios are in the reverse proportion. as, for example, if it is required
to know if six men can build a wall in four days how long it will take eight men to
build it;-this becomes
8: 6:4 : 3.
With my slide-rule I hold it so as to present to the eye the face of the instrument
which shows the graduations in a reversed order, B. 1. C I., and set son
scale C. I. against 4 on scale D, and under 8 on-scale C: I., I find 3 on scale D,
which is the answer required.
My improved rule is alsò of value in “performing two consecutive multiplications15

page_text: or divisions with one single setting of the slide. Thus, 2 x 3 x 4 = 24. This I
demonstrate thus:
That is to say, I set 2 on scale C. I.-C inverted, against 3 on scale D, and then
turning the rule over, I find under 4 on scale C (not inverted) the product, 24, of
the three factors, 2, 3 and 4.
On the ordinary slide-rule this requires two separate operations; the first, to
obtain the product of 2 x 3 = 6; and the second operation, with a second setting
of the slide, to obtain the product of 6 x 4 = 24.
It will therefore be seen that it is not necessary with my duplex slide-rule, as
with the ordinary slide-rule, to entirely remove the slide from the body and invert
it and return it to place, and then adjust it longitudinally to perform the problem,
because the two faces of my slide being both visible, and having the two faces of
the body both graduated alike, and the slide graduated on one face to correspond
with the body and on the other face with similar graduations in the reversed order,
it is only necessary to turn the entire instrument over and read from the other face
on the slide the graduations in the reversed order.
For further uses of my slide-rule see "The Slide-rule, by William Cox, published
by Keuffel & Esser Co., New York, 1891."
Having now particularly described and ascertained the nature of the said inven-
tion and in what manner the same is to be performed, I declare that what I
claim is:
1. In a slide-rule, in combination with a suitable graduated slide, the body
described composed of a lower bar, grooved on its upper edge and logarithmically
and identically graduated on each face, an upper bar grooved on its lower edge and
logarithmically and identically graduated on each face, the two bars rigidly
connected together by cross-pieces arranged to hold the lower and upper bars
firmly in their relative positions, as herein specified.
2. A double-faced slide-rule having a slide with lines of logarithmic graduations
on each face, in combination with a body composed of parallel rigidly connected
bars engaged with the edges respectively and graduated logarithmically on each
face and with the runner u enabling corresponding points on either face and on any
scale on either side to be indicated, as herein set forth.
3. The double-faced slide-rule described having lines of logarithmic graduations
on each face of the body and slide with some of the lines of graduations in direct
and others in reversed order adapted to overcome the necessity of inverting the
slide, as herein specified.
Dated this 6th day of October 1891.
ALLISON BROS.,
Agents for the Applicant.