Barometers:
Always when cruising carry a barometer, and constantly watch it. This instrument does not foretell the weather except indirectly. It is simply an instrument for measuring the weight of the atmosphere. No change of weather takes place without a change of the weight of the air; therefore, when the barometer remains steady there will be no change. If it goes up or down it means that some change will take place. By learning what usually follows such fluctuations you can employ it to help you in determining or foretelling the future weather conditions.