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NCERT Solutions Curiosity Chapter 8 .1 Measurement of Time — Activity 8.1: Let us construct

Book page 1078 Updated on2026-09-05

Q1.
Should we make a simple water clock?
Answer

Yes — and it needs nothing more than a used transparent plastic bottle. Here is the whole activity in order.

  1. Take a used transparent plastic bottle (½ litre or larger) with its cap.
  2. Cut it into two, roughly in the middle [Fig. 8.5a].
  3. Using a drawing pin, make a small hole in the cap [Fig. 8.5b].
  4. Place the upper part of the bottle, cap screwed on, inverted over the lower half [Fig. 8.5c].
  5. Fill the upper part with water. Add a few drops of ink or colour so that the water level is easy to see [Fig. 8.5d].
  6. The water starts dripping into the lower part. Using a watch, mark the level of water after every one minute till all the water has dripped down.
Why the marks are not equally spaced: as the water in the top half gets used up, the push of the water above the hole becomes less, so the dripping slows down a little. The same difficulty was faced in ancient India — the Arthasastra type of out-flow water clock was not very accurate because the flow rate decreased as the water level dropped. That is exactly why the sinking-bowl clock (Ghatika-yantra) was developed.
Check it yourself: make the hole with a single drawing pin, not a nail. A big hole empties the bottle in seconds and you get no useful marks.
Q2.
Your water clock is ready. Can you now guess how to use it?
Answer

Pour the water from the lower part back into the top part and watch the level of water dripping into the lower part. Every time the level touches a mark you had made, one more minute has passed.

So the clock is read by counting marks, not by looking at a dial:

  • Level at the 1st mark → 1 minute over.
  • Level at the 5th mark → 5 minutes over.
  • To time something — say how long a friend takes to solve 10 sums — start the water and note the mark reached when the friend finishes.
Why it works: the water always takes the same time to fall from one mark to the next as it did while you were making the marks, because the same bottle, the same hole and the same amounts of water are involved each time. A repeatable process is the heart of every clock.
Try this: use your water clock to time 10 oscillations of the pendulum you make in Activity 8.2, and compare it with a wrist watch. You will quickly see why people kept looking for better clocks.
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