685 OF ENGINEERING FORMULÆ,
Slide Rule—continued.
The “ gauge-point ” P = the number of units of the dimen-
sion contained in the unit of the answer. Thus if answer and
dinAnsions are all in feet, P — 1; for answer in feet with
dimensions F.I.I (feet X inches x indie.-), P = 1 X 12 x 12
= 144; for answer in inches with dimensions F. F.I,
P=j 1 X A- S= -00694; &C.
Oli some slide rules 1 A is opposite 4 D, in which case the
gauge point» for the ordinary slide rule must be divided by
16 Æ2). .
The slide rule is a proportional instrument, and a giveii case
should be reduced to the proportional form a •. b ■. : <c ■. x.
Terms a and b or e and x must never fall on the same line.
If A sind B be set to any proportion, then all the numbers
against each other in the same lines will be in the same ratio;
thus if the gauge point for the circumference of a circle on 15
ba set on 1 A, all the numbers on B will represent a table of
circumferences due to the diameter represented by the ligures
on A.
AVith the gauge point, P, for areas on B, set »gainst 1 A, tbo
scale C foiins a table of areas of circles for diameters repre-
sented by the figures on D. With I C set on 1 D, the scales C
and 1) are tables of squares and square roots respectively for
the numbers of the scales opposite to them.
Cases of x = a ™ or a » may be readily sol ved by the aid
of an equal parts scale applied to a slide-rule line. Thus for
V' lö3, if the distance from -16 to 1 of a slide rule measure
378 on a scale of 80, then 378 X = 22-7, and from 0 to 22'7
on the 80 scale reaches back from 1 tu • 333 = x on the slide-
rule scale.
Gauge Points on Roütlege’s Sude Rule.
Cub®
Ina.
Cul>« Gal-
Feet. ■ Ions.
Water 'Cast 'Brass C°P" J
" Iron. Iron. | | per.
Lead.
Tin.
F F F
FI I
578
88
F I
F
I
106
1273
1H6
191
144
1728
1833
191
33
163
231
277
249
353
306
521)
16 207
297 j
357 i
378 |
453
23
2765
293
352
305
628
222
32
384
407
489
3»4i 414
6t>2 I 733
193
218
333
354
424
369
637
18
26
312
331
397
345
596
141
203
258
31
27
465
219
5315
.378
401
481
429
723