estimate the height as between 5 ft. 6 in. and 5 ft. 8 in.; measurement
may show it to be between 5 ft. 6 in. and 5 ft. 7 in., but to go closer
than that requires many precautions. Training in observation and the use
of delicate instruments thus narrow the limits of approximation. Similarly
with regard to space and time, there are instruments with which one
millionth of an inch, or of a second, can be measured, but even this
approximation, although far closer than is ever practically necessary, is
not accuracy. In the statement of measurements there is no meaning in more
than six significant figures, and only the most careful observations can
be trusted so far. The height of Mount Everest is given as 29,002 feet;
but here the fifth figure is meaningless, the height of that mountain not
being known so accurately that two feet more or less would be detected.
Similarly, the radius of the earth is sometimes given as 3963.295833
miles, whereas no observation can get nearer the truth than 3963.30 miles.

--Mill: _The Realm of Nature_. (Copyright, 1892, by Charles Scribner's
Sons.)


4. The chief cause which made the fusion of the different elements of
society so imperfect was the extreme difficulty which our ancestors found
in passing from place to place. Of all the inventions, the alphabet and
the printing press alone excepted, those inventions which abridge distance
have done most for the civilization of our species. Every improvement of
the means of locomotion benefits mankind morally and intellectually as
well as materially, and not only facilitates the interchange of the
various productions of nature and art, but tends to remove national and
provincial prejudices, and to bind together all the branches of the great
human family. In the seventeenth century the inhabitants of London were
for almost every practical purpose farther from Reading than they are now