Modern meteorology was still in an embryological stage when the millers had attributed their true signification to the cirro-stratus clouds which stretched out in long, narrow bands, sometimes from the horizon to the zenith. They called them "wind trees." They were as well acquainted with the cirro-cumulus, the alto-stratus, and the cumulus. Proof of this is found in the enigmatical and obscure language of an old miller who declared he had seen in the sky "a shepherd under the shadow of a tree while the sheep were pasturing in the field."
Besides looking into the nature and meaning of the cirrus and the cirro-cumulus, the miller tried to calculate the force of the wind in distant storms; he observed the direction and velocity of the lower clouds; he estimated at sight the volume and density of the storm clouds; and if the wind fell off before the rain came, he recollected the old saying, "A calm comes before a storm."
He especially displays extraordinary vigilance in times of heavy showers. He observes, among other things, if the mills farthest away in the direction of the rain have kept their sails unfolded; if they have, it is a good sign. He scrutinizes the sky at every moment, from the zenith to the horizon; he measures the curvature of the forward part of the precipitation or of the storm. The lines of rain or hail that escape from it show him by their length and their approach to the perpendicular how intense the precipitations are. If the lines run obliquely, he is shown the direction of the dominant wind in the squall—in short, no sign indicating the force and direction of the wind passes unperceived by him. He knows likewise that these showers are often accompanied by tempestuous, plunging gusts which seem to come out of the clouds; and frequently, before the most advanced flecks of the storm cloud have reached the zenith, the sails of the mill are rolled up around the arms so as to give the squall free passage.
Except by meteorologists, it is still not generally known that the air is urged on more violently on the right of the squall than on the left; but the old miller knew it long ago by experience. The storms that passed by on his left, from west-by-southwest to south, never gave him any fear, and he confidently left his sails all unfurled in the wind; but whenever he was directly threatened with a storm which would pass over his zenith, or which was coming from the right—that is, from north-by-northeast to east, at a distance of less than five kilometres—he foresaw the possibility of a strong blow, and took his measures accordingly. I never knew of a miller who could account for the squalls from his left being less formidable than those from his right. The explanation of the phenomenon was reserved for modern meteorology.
