It is clear from the passage that the measurements of small units of time is ....... .

For centuries the sun's shadow remained the
universal measure of time. This was a handy
measure since a simple sundial could be made
anywhere by anybody without special knowledge or
equipment. But there was an obvious limitation. A
sundial measures the sun's shadow: no sun, no
shadow. A shadow clock was useful only in those
parts of the world where there was lots of sunlight,
and then it served only when the sun was actually
shining. Even when the sun shone brightly, the
movement of the sun's shadow was so slow that it
would be little help in marking minutes, and useless
for signaling seconds.