第57章
Recent castings naturally flow down an inclined surface, but on a grassy field, which sloped between 10 degrees and 15 degrees, several were found after a heavy gale blown up the slope.This likewise occurred on another occasion on a part of my lawn where the slope was somewhat less.On a third occasion, the castings on the steep, grass-covered sides of a valley, down which a gale had blown, were directed obliquely instead of straight down the slope; and this was obviously due to the combined action of the wind and gravity.Four castings on my lawn, where the downward inclination was 0 degrees 45 seconds, 1 degree, 3 degrees and 3 degrees 30 seconds (mean 2 degrees 45 seconds) towards the north- east, after a heavy south-west gale with rain, were divided across the mouths of the burrows and weighed in the manner formerly described.The mean weight of the earth below the mouths of burrows and to leeward, was to that above the mouths and on the windward side as 2.75 to 1; whereas we have seen that with several castings which had flowed down slopes having a mean inclination of 9 degrees 26 seconds, and with three castings where the inclination was above 12 degrees; the proportional weight of the earth below to that above the burrows was as only 2 to 1.These several cases show how efficiently gales of wind accompanied by rain act in displacing recently ejected castings.We may therefore conclude that even a moderately strong wind will produce some slight effect on them.
Dry and indurated castings, after their disintegration into small fragments or pellets, are sometimes, probably often, blown by a strong wind to leeward.This was observed on four occasions, but I did not sufficiently attend to this point.One old casting on a gently sloping bank was blown quite away by a strong south-west wind.Dr.King believes that the wind removes the greater part of the old crumbling castings near Nice.Several old castings on my lawn were marked with pins and protected from any disturbance.They were examined after an interval of 10 weeks, during which time the weather had been alternately dry and rainy.Some, which were of a yellowish colour had been washed almost completely away, as could be seen by the colour of the surrounding ground.Others had completely disappeared, and these no doubt had been blown away.Lastly, others still remained and would long remain, as blades of grass hadgrown through them.On poor pasture-land, which has never been rolled and has not been much trampled on by animals, the whole surface is sometimes dotted with little pimples, through and on which grass grows; and these pimples consist of old worm- castings.
In all the many observed cases of soft castings blown to leeward, this had been effected by strong winds accompanied by rain.As such winds in England generally blow from the south and south-west, earth must on the whole tend to travel over our fields in a north and north-east direction.This fact is interesting, because it might be thought that none could be removed from a level, grass- covered surface by any means.In thick and level woods, protected from the wind, castings will never be removed as long as the wood lasts; and mould will here tend to accumulate to the depth at which worms can work.I tried to procure evidence as to how much mould is blown, whilst in the state of castings, by our wet southern gales to the north-east, over open and flat land, by looking to the level of the surface on opposite sides of old trees and hedge-rows; but I failed owing to the unequal growth of the roots of trees and to most pasture-land having been formerly ploughed.
On an open plain near Stonehenge, there exist shallow circular trenches, with a low embankment outside, surrounding level spaces 50 yards in diameter.These rings appear very ancient, and are believed to be contemporaneous with the Druidical stones.Castings ejected within these circular spaces, if blown to the north-east by south-west winds would form a layer of mould within the trench, thicker on the north-eastern than on any other side.But the site was not favourable for the action of worms, for the mould over the surrounding Chalk formation with flints, was only 3.37 inches in thickness, from a mean of six observations made at a distance of 10 yards outside the embankment.The thickness of the mould within two of the circular trenches was measured every 5 yards all round, on the inner sides near the bottom.My son Horace protracted these measurements on paper; and though the curved line representing the thickness of the mould was extremely irregular, yet in both diagrams it could be seen to be thicker on the north-eastern side than elsewhere.When a mean of all the measurements in both the trenches was laid down and the line smoothed, itwas obvious that the mould was thickest in the quarter of the circle between north- west and north-east; and thinnest in the quarter between south-east and south-west, especially at this latter point.Besides the foregoing measurements, six others were taken near together in one of the circular trenches, on the north-east side; and the mould here had a mean thickness of 2.29 inches; while the mean of six other measurements on the south-west side was only 1.46 inches.These observations indicate that the castings had been blown by the south-west winds from the circular enclosed space into the trench on the north-east side; but many more measurements in other analogous cases would be requisite for a trustworthy result.