“I have done so.”
“Then now blow it out.”
“I cannot imagine what you are about, papa;--‘Light the candle, and then blow it out!’--but it still smokes, shall I put the extinguisher over it?”
“By no means; give it to me, and observe what will happen when I carry it round the edge of the dish.”
“The smoke goes to the centre,” exclaimed Tom.
“Showing, thereby, the existence of a current towards the water-plate, or island; in consequence of the air above it having been heated, and therefore rarefied. This explains in a very satisfactory manner, a fact which may be constantly observed in our own climate, viz. a gentle breeze blowing from the sea to the land in the heat of the day. Upon the same principle it is, that most of the winds in different parts of the globe may be readily accounted for.”[(42)]
“I suppose,” said Tom, “that the air must rush with great velocity, in order to produce wind.”
“A very general error prevails upon this subject,” replied his father; “the rate of motion has been greatly exaggerated. In a brisk gale, even, the wind does not travel with such velocity, but that it may be easily traced by the eye; and the sailor is able to watch its progress by the ripple which it produces on the sea.”
“Has, then, the rate of its motion ever been estimated?” asked Louisa.
“When its velocity is about two miles per hour, it is only just perceptible. In a high wind, the air travels thirty or forty miles in the same period. In a storm, its rate has been computed as being from sixty to eighty miles. It has also been ascertained, by experiment, that the air, as it rushes from a pair of blacksmith’s bellows, has not a velocity above that of five and forty miles in the hour.”