Book V — Which Treats of Winds and Atmospheric Movement in General
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Wind is the atmosphere in motion. Some have put the definition thus: Wind is the atmosphere in motion in one direction. The latter seems the more accurate, because the atmosphere is never so still as not to be in agitation of some kind. In a similar way the sea is called calm when it is only slightly moved and does not set in a particular direction. Thus, if you read the verse:
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When the winds slumbered and the sea was still, you must bear in mind that the sea was not actually still, but heaved gently; and that it is called calm in a comparative sort of way because it receives no distinct impulse to this side or to that. The same opinion is likewise to be adopted in regard to the atmosphere: it is never absolutely motionless, even though it be still. This you may gather from the following observation: When the sun pours into any circumscribed space, one sees minute particles carried through the air in different directions, some up, some down, meeting each other in a great variety of ways. Therefore, if one say: a wave is an agitation of the sea, one will very imperfectly express what is meant, because even when at rest the sea is agitated. But one will more than sufficiently safeguard oneself if the definition be: a wave is an agitation of the sea in one direction. So in the subject which at the moment forms our special topic, the definition will not be unduly restricted if one is careful to say: wind is the atmosphere flowing in one direction; or, wind is atmosphere flowing through some impulse, or, is the force of the atmosphere going in one direction, or, is a rush of the atmosphere more forcible than usual in some one direction. I am aware of a criticism that may be made in regard to the first definition. What need is there to add that it is in one direction that the atmosphere flows? For surely whatever flows, flows in one direction. No one says that water flows if there is simply an internal movement of it, but only if it is borne in a particular direction. So a substance may be in motion and yet not flow; but, on the other hand, it cannot flow except in one direction. Well, if, on the one hand, the shorter definition is free from cavil, let us employ it; but if, on the other, any one is a stickler, let him not omit the phrase whose addition will serve to preclude all ambiguity. Now that we have sufficiently discussed our terms, let us come to grapple with our problem at closer quarters.