Omaha daily bee., 1897-03-07

ratus is so much more delicate that with a
sufficiently strong telescope placed about %
feet away 1t would be possible to read down
to 1900, 000th of an inch, and there would
be interval enough between the graduations
to make an accurate gusss of LOCO, 000th of
an inch.
MEASURING SMALL FRACTIONS.
In the manufacture of such instruments as
these, other instruments, hardly less deli
cate and accurate, are In turn required.
Take what is known as a, micrometer caliper,
on which accurate measurements may be
made to Ito, 000th of an Inch. 1t is easy to
see that the slightest inaccuracy in any one
OF its parts would throw all the others out
of adjustment and destroy the value of the
instrument. Expensive &! special machinery
of almost perfect precision is necessarily
required for this class ofbwork, and the final
testing of a caliper of fhe kind mentioned
must be made with a Standard scale. The
latest development OF this type of appli-
ances is to be seen in the shops of a manu-
featuring company in Providence, K. j. On
a massive bed, eighteen inches high, are two.
movable heads, fitted to the broad, fiat surface
and gibbed at the sides. The larger head
carries a bar having a finely graduated
scale, with markings down to a 10th of an
inch on the upper side and another on
the lower side, so fine as to be invisible
| without the aid of a powerful glass. Over
this latter scale is a microscope fitted with
a micrcmeter eye-piece, for reading these
fine gradations. The latter are read by
means of a vernier scale, and their valuation
Is O.00001 of an inch. The cone at the back
Of the machine is for the purpose of con-
centrating light upon tha graduation of the
scale.
The chief adjustment bf the machine is
by the manipulation lof the hair lines of
the microscope so that the latter are coin
cident with the lines Of the scale. Thus, in
taking a measurement the machine Is first
adjusted to a point on the graduating scale
corresponding to the nearest fortieth of an
inch to the size to be measured. Intervals
OF less than this are obtained by means
OF the mlcrometer screw. Here. likewise, for
fne readings. care must be taken to pro-
vide a constant temperature, since 1t seems
impossible to construct IAN instrument of
perfect compensation for THE different rates
or expansion of the various metals compos-
ing it.
Intervals of space have been measured
for a long while, but minute intervals OF
time offered a more complex problem. From
a physical point Of view. the time unit,
unlike the units OF length or mass, is in
a sense a derived unit. Unlike the stand
ard meter or gram, which can be directly
examined, compared ana copied, we have
no direct model Of a second, but must de-
rive the unit by observations upon the mo-
ton OF some standard body, such as a pen
dulum or the earth, This process Of observ-
ing the motion of The standard body involves
the recording of the exact instant between
two or more points " the motion of the
body which successfully repeat themselves.
Ultimately, then, the reliability OF such
measurement depends upon ability to pro-
duce a record practically simultaneous
with the occurrence OF the phenomena itself.
The two practically Instantaneous modes of
motion we have are light and electricity.
The old ehronograph, used for the measure
ment of the speed Of projectiles, was an
electrical device, In which a series of wire
screens were placed in the path of the pro-
jeetile at interYals. so that the latter would
cut the screens and thus break the cir
cult. The firing ef the gun likewise caused
the release OF a bar. which. as 1t fell. passed
flight cut the successive wires, the breaks
in the circuits caused these knlyed to fly
out and lightly mark the failing bar. Thus,