Consider the following statements
1. Glaciers always melt at the bottom first.
2. Calorimeters are generally made of coppers.
3. The unit of latent heat is cal/g.
4. If a substance contracts on heating, its co-efficient of linear expansion is negative.
Which of the statements given above are correct?

Consider the following statements:

1. A heat engine is a device which converts heat energy into mechanical energy.

2. A heat engine essentially consists of a source of heat at a higher temperature, working substance and sink of heat at a lower temperature.

3. In a diesel engine, a mixture of fuel vapours and air acts as the working substance.

4. The efficiency of a petrol engine is greater than the diesel engine.

Which of the statements given above is/are correct?