_STANDARD MEASURES._
Machines are composed of parts connected together by rigid and
movable joints; rigid joints are necessary because of the expense,
and in most cases the impossibility, of constructing framing and
other fixed detail in one piece.
All moving parts must of course be independent of fixed parts, the
relation between the two being maintained by what has been called
running joints.
It is evident that when the parts of a machine are joined
together, each piece which has contact on more than one side must
have specific dimensions; it is farther evident that as many of
the joints in a machine as are to accommodate the exigencies
of construction must be without space, that is, they represent
continued sections of what should be solid material, if it were
possible to construct the parts in that manner. This also demands
specific dimensions.
In arranging the details of machines, it is impossible to have a
special standard of dimensions for each case, or even for each
shop; the dimensions employed are therefore made to conform to some
general standard, which by custom becomes known and familiar to
workmen and to a country, or as we may now say to all countries.
A standard of lineal measures, however, cannot be taken from one
country to another, or even transferred from one shop to another
without the risk of variation; and it is therefore necessary
that such a standard be based upon something in nature to which
reference can be made in cases of doubt.
In ages past, various attempts were made to find some constant in
nature on which measures could be based. Some of these attempts
were ludicrous, and all of them failures, until the vibrations of
a pendulum connected length and space with time. The problem was
then more easy. The changes of seasons and the movement of heavenly
bodies had established measures of time, so that days, hours, and
minutes became constants, proved and maintained by the unerring
laws of nature.
A pendulum vibrating in uniform time regardless of distance, but
always as its length, if arranged to perform one vibration in a
given time, gave a constant measure of length. Thus lineal measure
comes from time; cubic or solid measures from lineal measure, and
standards of weight from the same source; because when a certain
quantity of a substance of any kind could be determined by lineal
measurement, and this quantity was weighed, a standard of weight
would be reached, provided there was some substance sufficiently
uniform, to which reference could be made in different countries.
Such a substance is sea or pure water; weighed in vacuo, or with
the air at an assumed density, water gives a result constant
enough for a standard of weight.
It is a strange thought that with all the order, system, and
regularity, existing in nature, there is nothing but the movements
of the heavenly bodies constant enough to form a base for gauging
tests. The French standard based upon the calculated length of the
meridian may be traced to this source.
Nothing animate or inanimate in nature is uniform; plants,
trees, animals, are all different; even the air we breathe and
the temperature around us is constantly changing; only one thing
is constant, that is time, and to this must we go for all our
standards.
I am not aware that the derivation of our standard measures has
been, in an historical way, as the foregoing remarks will indicate,
nor is it the purpose here to follow such history. A reader, whose
attention is directed to the subject, will find no trouble in
tracing the matter from other sources. The present object is to
show what a wonderful series of connections can be traced from so
simple a tool as a measuring gauge, and how abstruse, in fact,
are many apparently simple things, often regarded as not worth a
thought beyond their practical application.
(1.) Why are machine frames constructed in sections, instead of
being in one piece?--(2.) Why must parts which have contact on
opposite sides have specific dimensions?--(3.) What are standards
of measure based upon in England, America, and France?--(4.) How
can weight be measured by time?--(5.) Has the French metre proved
a standard admitting of test reference?