victor steinbok aardvark66 at GMAIL.COM
Tue May 3 11:33:04 UTC 2011

Some observations on compasses and calipers, although my experience with
these is by no means universal (and, I am sure, under many circumstances can
be seen as wrong or incomplete)

I came across a Supreme Court phrase that I wanted to check and looked up
"compass" in the OED.

In addition to bringing possession *within the statute’s compass*, Congress
> increased the severity of §924(c) sentences by changing “once mandatory
> sentences into mandatory minimum sentences,” United States v. O’Brien, 560
> U. S. ___, ___, and by elevating the sentences for brandishing and
> discharging a firearm and for repeat offenses. Congress also restructured
> the provision, “divid[ing] what was once a lengthy principal sentence into
> separate subparagraphs,” id., at ___, and it added the “except” clause at
> issue. Pp. 5–8.

[emphasis added]

There is nothing new or noteworthy in the SCOTUS sentence, but as I was
reading the definitions, I came across this description for "Mathematical

4.a. An instrument for taking measurements and describing circles,
> consisting (in its simplest form) of two straight and equal legs connected
> at one end by a movable joint. *Now gen. in pl.; also pair of compasses.*

Modifications of this instrument are
> the bow-compasses (see bow-compass n.); beam-, calliper- hair-compasses,
> etc. Similar instruments for describing figures other than circles are
> specified by a corresponding adj., aselliptic, oval, triangular compasses;
> also proportional compasses: see these adjs.

[Emphasis added, where formatting has otherwise been stripped.]

There are two sets of examples attached to this definition, terminating,
respectively, in 1841 and 1840. The first set is examples with singular
"compass", the second with plural "compasses". I have no problem stating
both possibilities, but I take issue with the claim "Now gen. in pl.". What
time period is described by "Now"? The 1840s?

On the other hand, under "Calliper", the first definition describes
"calliper compasses":

1.a. Originally used attrib., calliper compasses or compasses calliper,
> compasses used to measure the calibre of shot; *afterwards usually in
> pl. callipers,pair of callipers*: A kind of compasses with bowed legs for
> measuring the diameter of convex bodies; often with a scale attached for
> reading off the measurements; also a similar instrument with straight legs
> and points turned outwards for measuring the bore or internal diameter of
> tubes, etc.

 b. Applied to measuring rules of varying shape for taking the dimensions of
> other than round bodies.   calliper-square n. a rule or square carrying
> movable cross-heads, adapted for the measurement of internal and external
> diameters or sizes.

 [Emphasis added, where formatting has otherwise been stripped.]

Again, the claim is that it's "usually in pl." Here the issue is somewhat
different because the definition (and citations) has not been upgraded since

Both seem wrong but for entirely different reasons.

One would be hard-pressed to find a commercial product called "Compasses".
And you will never see or hear "student compasses", except in reference to
multiple instantiations of such products. In fact, most "mathematical
compasses" today are such "student compasses". Most office stores will offer
multiple versions of a "student compass" and one or two versions of a
"professional compass". Most are still instances of a "bow compass", but the
description is usually just limited to a single word. Some are the
prototypical kind, either with an adjustment wheel or without (some place
the "wheel" on top rather than in the middle).

Here are a couple of examples of a different kind:

I'm not entirely sure if the original "student compass"--the "retarded" bow
compass with one leg replaced by a loop to hold a pencil or a pen--is an
example of a "bow compass". But it's a fairly standard product that falls
squarely under the definition cited above. But it certainly does not "now
generally" appear in plural.

You also will never hear the plural version of "construction with
straightedge and compass". In fact, having more than one compass would have
a somewhat different implication, although one that has little effect on the
outcome of such constructions. In general, in school settings, I doubt there
is much talk of "compasses"--at least, in the US. Don't know about the rest
of the Anglophones.

I am not sure when and under what circumstances "calliper" lost one of its
ls. But most references today--in US, at least--use "caliper". Typical
"caliper" is pictured here:

Often referred to as "vernier caliper", the slightly more traditional kind
has a nomographic sliding scale rather than a digital display--a kind of a
primitive slide-rule for measurements. Another version uses a dial. This
particular kind of universal/slide caliper measures both interiors (smaller
upper jaws) and exteriors (lower large jaws) of cylindrical/tubular objects,
depths of holes (the slide-out depth-probe), as well as just general
distances and sizes. However, these do not match either 1.a. or 1.b.--the
former only applies to "compasses" (see immediately below) and the latter is
not meant merely "for taking the dimensions of other than round
bodies"--there is absolutely nothing wrong with using vernier calipers for
measuring round or spherical bodies. In fact, all three examples under 1.b.
refer to slide calipers for measuring diameters--something vernier calipers
have always been uniquely suited for!

1708    J. Kersey Dict. Anglo-Britannicum,   Callipers, an instrument made
> like a Sliding-Rule, to embrace the two Heads of a Cask, or Barrel, in order
> to find the length of it.

1876    Catal. Sci. Appar. S. Kens. No. 293,   Collection of Timber
> Callipers for the use of foresters.
> 1888    N.E.D. at Calliper,   Mod. techn. Calliper (in Liverpool timber
> yards), a rule for measuring timber, something like that which shoemakers
> use to measure feet.

The older styles of cal(l)ipers are inside/outside calipers pictured in Wiki

These two and vernier calipers can be seen here

These look like primitive compasses with bent legs and are still in use for
quick comparisons where the vernier calipers are impractical. These are
exactly the two kinds of "calliper compasses" that are described under
caliper 1.a. (see above).

I often see references to each of these as "calipers" rather than singular
"caliper", although both appear to be in use. I saw these recently in a
local lumber store with plural "calipers" on the package. But the Wiki
article uses singular "caliper" throughout.

To make matters slightly worse, the latest (1876) example under 1.a.
actually refers to patented vernier caliper and should appear instead under

1876    Catal. Sci. Appar. S. Kens. No. 284,   Universal Calliper, with
> slide and reverse action. No. 271 Calliper with Dial‥divided into eighths of
> an inch.

The fourth type of calipers is "divider caliper" (also in Wiki), which is
normally used just as "divider". These are usually seen in movies when
characters are measuring distances on marine maps. But I'm used to these as
separate student implements occasionally confused with compasses--a divider
is essentially a compass with the graphite lead replaced with a second
needle, although many are integral. To be honest, I can't think of anyone
referring to one of these as "cal(l)iper(s)", so I can't really say with any
certainty whether these appear as singular or plural when they are referred
to as such.

Finally, there is also a micrometer caliper, usually referred to as
"micrometer" alone or "micro-caliper" or "micro[meter]-gauge" (also in Wiki
under "micrometer"). These are structurally different from the
universal/slide calipers as they use a handle-based thumb-screw to tighten
the measuring jaws on the unsuspecting part. I've also heard references to
both "caliper" and "calipers" for micrometers in shops and labs.

Whatever the case, both OED claims that "[mathematical] compass" and
cal(l)iper" are "now" and "usually" used as plurals appear to be
significantly overstated, at best--at least, when it comes to US usage.
British usage might be similar to the math/maths distinction. I have no
idea. I suspect, I would have found more references to such had it been the
case. So the definitions for both need adjustment and expansion--they both
seem to be stuck in something like 1876.

Another odd thing is that "compass" in the sense of "compass rose" is not
listed. Nor, in fact, is there a separate entry for "compass rose" at all.
The only mention is under rose 13.a.

> 13.a. A circular pattern showing the thirty-two points of the
> compass; spec. the card of a mariner's compass (now usu. compass rose) or of
> a barometer. Cf. wind-rosen. 2.

If it's "now usu. compass rose", should the "compass rose" have a separate
entry or, at least, a compounds under "compass"? And, in fact, references to
it as a "compass" (as in, a picture of a compass) rather than a "rose" are
far more common. So the omission is puzzling.


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