It is commonly considered that the particles of moisture within a cloud are quite motionless; and when looking at a huge cloud floating serenely in a summer sky it is difficult not to think of its constituent parts as being quite at rest. The apparently stationary cloud is all commotion and movement, the particles within it being always on the move, some going up and others down. The particles of moisture, moreover, being probably only about the four-thousandth part of an inch in diameter, the resistance offered by the air to their movement is very slight; indeed, as soon as they are condensed they immediately begin to fall downward, and were it not for the atoms of dust waiting to catch them the particles would at once fall to the ground. It is often asked why the vapor, if so readily condensed in the atmosphere, does not continually fall to the earth. The answer to this question, it will be seen, is that the moisture, instead of always pouring down on the earth, settles on the surface of the atoms of dust. Thus the first downward movement of the incipient raindrop is arrested by the dust-nuclei which swarm in all parts of the atmosphere; so that instead of being destroyed as soon as it is formed, the particle of moisture is preserved and stored for future use. In realizing the fact that a cloud is always in motion, the first step has been taken in discovering how a raindrop is formed. It might be supposed that the raindrops would evaporate as quickly as they were condensed; but observation of the drops of moisture running down a window-pane and forming larger drops gives a good idea of what occurs in the clouds; as also does the fact that in a bottle of soda-water the bubbles of air overtake one another and, colliding, make larger bubbles. One of the principal causes of the manufacture of a raindrop is to be found in the circumstance that there is a similar process of amalgamation at work in every part of the atmosphere. It often happens that a drop of moisture falls downward through a cloud for a distance of a mile or more; and although it may pass through strata of very warm air, thus running a great risk of being evaporated and destroyed, it has also many collisions by which its bulk is considerably increased, and eventually becomes so heavy that its rate of progress is very much accelerated. Then, no longer able to float in the air, it plumps down to the earth as a full-grown raindrop.