How winds are caused we shall see by the aid of the diagram on the left-hand side of the illustration.
The original cause is the sun. His rays fall upon the earth, heating it, and so by radiation heating the air. Now, as has been remarked, heated air will cause a heavy balloon to float through ordinary air, and to carry up a considerable amount of dead weight besides; consequently the heated air must ascend, while cool and heavier air rushes in to take its place, and thus the currents are produced. Were the earth set straight upright, the currents would invariably run in one direction; but, as it is tilted on one side, the needful variety is obtained, and we find the winds blowing from all parts of the compass.
The principle, therefore, of all winds is, that heat expands, and therefore becomes lighter than air at an ordinary temperature.
Were it not that man has taken advantage of this principle, there could not be a deep mine in England. In any deep excavation, even though it be a well, foul air, mostly composed of carbonic acid gas, always collects, and, being much heavier than atmospheric air, lies at the bottom of the pit as surely as hydrogen would rise out of it. To breathe this air is as certain and as sudden death as to take prussic acid, and no mine can be worked as long as “choke-damp” is in it.
In coal mines there is an additional source of danger, namely, the coal gas, which is nearly identical with our coal gas of the streets, and takes fire when brought into contact with flame. To rid the mines of these gases, a simple, ingenious, and effectual remedy is used. A ventilating shaft is made, which reaches from the bottom to the mouth of the pit. At the bottom, diagonal shafts are made, entering the main shaft, as shown on the right hand of the illustration. One of these is connected with a furnace, and the other, or others, open into the mine.
The heat of the furnace rarefies the air in the shaft, causing it to rush upwards with great violence, and so, by creating a partial vacuum, to force the air in the shaft to follow it. The loss of air thus caused is supplied by fresh air from above, which, by the law already described, is obliged to take the place of that which was driven out. Thus a complete circulation of air is kept up, and a well-managed mine has a fresher atmosphere than many houses in which the windows are mostly kept shut, and the only ventilation is accomplished by occasionally open doors.
The “draught” of our domestic chimneys is owing to this principle, and the reason why factory chimneys are built of such enormous height is, that the column of heated air may be increased, and consequently that the draught may be stronger, and the heat of the furnace made fiercer.
The “Steam-blast,” by which the escape steam of engines is sent into the chimney, is another example of this principle, the steam taking the place of the hot air.
Further examples of the weight of the atmosphere are given in the illustration. That on the right represents the common Wheel Barometer, which marks the weight of the air by a hand moving in front of a dial. If the hand moves towards the right, the weight of the air is increasing; if to the left, it is decreasing.