The compass

Reshma's sailors cannot see the stars in the storm. Travellers in such times used a magnetic compass. It works because a freely turning magnet always rests north-south.

A compass is usually a small round box with a clear cover. Inside, a magnet shaped like a needle is balanced on a pin so that it can turn freely. The end that rests towards the north is usually painted red. Below the needle is a dial with the directions marked on it.

To use it:

• Keep the compass where you want to find the directions.
• Wait for the needle to come to rest north-south.
• Gently turn the box until the N and S on the dial line up with the needle.
• Now the dial shows all the directions at that place.

You can make a compass yourself:

• Lay an iron sewing needle on a wooden table.
• Put one pole of a bar magnet at one end of the needle and slide it along to the other end. Lift it up, bring the same pole back to the same starting end, and slide again.
• Always rub in one direction only, 30 to 40 times. Now the needle is a magnet: it attracts steel pins or iron filings.
• Push the needle through a cork and float the cork in a bowl of water. When it comes to rest, your compass is ready: its ends point the same way every time.

Long before the modern compass was common, Indian sailors used a device like this at sea: a magnetised iron piece shaped like a fish, kept in a vessel of oil. It was called the matsya-yantra (or machchh-yantra).