The two great Forces of Nature are Gravitation and Heat, which always act in opposition to each other.

Weather is the result of the action of these forces on matter, and where one form of force is in excess of another, changes are produced which become apparent to our senses, or are indicated by suitable instruments.

The Matter composing the earth on which we live is of three kinds—solid, liquid, and gaseous.

The Force incessantly acting on these is the radiant heat of the sun.

The Results of this incessant action are:—

1. Calorification, or Heating, which, besides being appreciable by our senses, is indicated by the Thermometer.

2. Evaporation, which alters the weight of the air indirectly, by the diffusion of aqueous vapour through it. This alteration of weight is indicated by the Barometer, the accompanying increase of moisture being indicated by the Hygrometer.

3. Rarefaction, which alters the weight of the air directly.

4. Condensation, producing fog, dew, rain, hail, and snow; all sufficiently apparent when they occur, but estimated accurately only by the Rain Gauge, or Pluviometer.

5. Motion, producing winds, which we are able to appreciate in the gentle breeze and the awful cyclone, the force and velocity of which are indicated by the Anemometer.

6. Electrification, producing lightning, thunder, magnetic phenomena, and chemical change, respectively indicated by the Electrometer, Magnetometer, and Ozonometer.