Pennsylvania, Dec. 10, 1884.]

By JOHN A. BRASHEAR.


In our study of the exact methods of measurement in use to-day, in the
various branches of scientific investigation, we should not forget that it
has been a plant of very slow growth, and it is interesting indeed to
glance along the pathway of the past to see how step by step our micron of
to-day has been evolved from the cubit, the hand's breadth, the span, and,
if you please, the barleycorn of our schoolboy days. It would also be a
pleasant task to investigate the properties of the gnomon of the Chinese,
Egyptians, and Peruvians, the scarphie of Eratosthenes, the astrolabe of
Hipparchus, the parallactic rules of Ptolemy, Regimontanus Purbach, and
Walther, the sextants and quadrants of Tycho Brahe, and the modifications
of these various instruments, the invention and use of which, from century
to century, bringing us at last to the telescopic age, or the days of
Lippershay, Jannsen, and Galileo.

[Illustration: FIG. 1.]

It would also be a most pleasant task to follow the evolution of our
subject in the new era of investigation ushered in by the invention of
that marvelous instrument, the telescope, followed closely by the work of
Kepler, Scheiner, Cassini, Huyghens, Newton, Digges, Nonius, Vernier,
Hall, Dollond, Herschel, Short, Bird, Ramsden, Troughton, Smeaton,
Fraunhofer, and a host of others, each of whom has contributed a noble
share in the elimination of sources of error, until to-day we are
satisfied only with units of measurement of the most exact and refined
nature. Although it would be pleasant to review the work of these past