How does the mercalli scale measure an earthquake?

The Mercalli scale measures the intensity of an earthquake. It measures the effects of an earthquake, and is distinct from the moment magnitude usually reported for an earthquake.

From A scale of 1 top 12.

Explanation:

The intensity s measured by the

1. Not felt.  --- magnitude of 1.0

2. Weak.few people. The  magnitude of 1.0 to 3.0.  

3. Weak few people. The  magnitude of 3.0 to 3.9.

4. Light felt during the day. The  magnitude of 4.0–4.9

5. Moderate mostly felt. The  magnitude of 4.0–4.9.

6. Strong, felt by all.      (5.0–5.9)

7. Very strong damage is negotiable to buildings.  (5.0–5.9)

8. Severe, heavy furniture overturned. The  magnitude of 6.0–6.9.

9. Violent, great damage in substantial buildings. (above 7.0)

10. Extreme, railways lines bent.  

11. Extreme,  bridges destroyed.

12. Extreme,  total; damage and destruction.