the formation of vegetable mould-第32节

按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！


annually　flows　down　a　slope　having　a　mean　inclination　of　9　degrees　
26　seconds。　　This　was　done　by　my　son　George。　　It　has　been　shown　
that　almost　exactly　two…thirds　of　the　ejected　earth　is　found　below　
the　mouth　of　the　burrow　and　one…third　above　it。　　Now　if　the　two…
thirds　which　is　below　the　hole　be　divided　into　two　equal　parts；　the　
upper　half　of　this　two…thirds　exactly　counterbalances　the　one…third　
which　is　above　the　hole；　so　that　as　far　as　regards　the　one…third　
above　and　the　upper　half　of　the　two…thirds　below；　there　is　no　flow　
of　earth　down　the　hill…side。　　The　earth　constituting　the　lower　half　
of　the　two…thirds　is；　however；　displaced　through　distances　which　
are　different　for　every　part　of　it；　but　which　may　be　represented　by　
the　distance　between　the　middle　point　of　the　lower　half　of　the　two…
thirds　and　the　hole。　　So　that　the　average　distance　of　displacement　
is　a　half　of　the　whole　length　of　the　worm…casting。　　Now　the　average　
length　of　ten　out　of　the　above　eleven　castings　was　2。03　inches；　and　
half　of　this　we　may　take　as　being　1　inch。　　It　may　therefore　be　
concluded　that　one…third　of　the　whole　earth　brought　to　the　surface　
was　in　these　cases　carried　down　the　slope　through　1　inch。　｛77｝

It　was　shown　in　the　third　chapter　that　on　Leith　Hill　Common；　dry　
earth　weighing　at　least　7。453　lbs。　was　brought　up　by　worms　to　the　
surface　on　a　square　yard　in　the　course　of　a　year。　　If　a　square　yard　
be　drawn　on　a　hillside　with　two　of　its　sides　horizontal；　then　it　is　
clear　that　only　1/36　part　of　the　earth　brought　up　on　that　square　
yard　would　be　near　enough　to　its　lower　side　to　cross　it；　supposing　
the　displacement　of　the　earth　to　be　through　one　inch。　　But　it　
appears　that　only　of　the　earth　brought　up　can　be　considered　to　flow　
downwards；　hence　1/3　of　1/36　or　1/108　of　7。453　lbs。　will　cross　the　
lower　side　of　our　square　yard　in　a　year。　　Now　1/108　of　7。453　lbs。　
is　1。1　oz。　　Therefore　1。1　oz。　of　dry　earth　will　annually　cross　each　
linear　yard　running　horizontally　along　a　slope　having　the　above　
inclination；　or　very　nearly　7　lbs。　will　annually　cross　a　horizontal　
line；　100　yards　in　length；　on　a　hill…side　having　this　inclination。

A　more　accurate；　though　still　very　rough；　calculation　can　be　made　
of　the　bulk　of　earth；　which　in　its　natural　damp　state　annually　
flows　down　the　same　slope　over　a　yard…line　drawn　horizontally　
across　it。　　From　the　several　cases　given　in　the　third　chapter；　it　
is　known　that　the　castings　annually　brought　to　the　surface　on　a　
square　yard；　if　uniformly　spread　out　would　form　a　layer　0。2　of　an　
inch　in　thickness：　　it　therefore　follows　by　a　calculation　similar　
to　the　one　already　given；　that　1/3　of　0。2x36；　or　2。4　cubic　inches　
of　damp　earth　will　annually　cross　a　horizontal　line　one　yard　in　
length　on　a　hillside　with　the　above　inclination。　　This　bulk　of　damp　
castings　was　found　to　weigh　1。85　oz。　　Therefore　11。56　lbs。　of　damp　
earth；　instead　of　7　lbs。　of　dry　earth　as　by　the　former　calculation；　
would　annually　cross　a　line　100　yards　in　length　on　our　inclined　
surface。

In　these　calculations　it　has　been　assumed　that　the　castings　flow　a　
short　distance　downwards　during　the　whole　year；　but　this　occurs　
only　with　those　ejected　during　or　shortly　before　rain；　so　that　the　
above　results　are　thus　far　exaggerated。　　On　the　other　hand；　during　
rain　much　of　the　finest　earth　is　washed　to　a　considerable　distance　
from　the　castings；　even　where　the　slope　is　an　extremely　gentle　one；　
and　is　thus　wholly　lost　as　far　as　the　above　calculations　are　
concerned。　　Castings　ejected　during　dry　weather　and　which　have　set　
hard；　lose　in　the　same　manner　a　considerable　quantity　of　fine　
earth。　　Dried　castings；　moreover；　are　apt　to　disintegrate　into　
little　pellets；　which　often　roll　or　are　blown　down　any　inclined　
surface。　　Therefore　the　above　result；　namely；　that　24　cubic　inches　
of　earth　（weighing　1。85　oz。　whilst　damp）　annually　crosses　a　yard…
line　of　the　specified　kind；　is　probably　not　much　if　at　all　
exaggerated。

This　amount　is　small；　but　we　should　bear　in　mind　how　many　branching　
valleys　intersect　most　countries；　the　whole　length　of　which　must　be　
very　great；　and　that　earth　is　steadily　travelling　down　both　turf…
covered　sides　of　each　valley。　　For　every　100　yards　in　length　in　a　
valley　with　sides　sloping　as　in　the　foregoing　cases；　480　cubic　
inches　of　damp　earth；　weighing　above　23　pounds；　will　annually　reach　
the　bottom。　　Here　a　thick　bed　of　alluvium　will　accumulate；　ready　to　
be　washed　away　in　the　course　of　centuries；　as　the　stream　in　the　
middle　meanders　from　side　to　side。

If　it　could　be　shown　that　worms　generally　excavate　their　burrows　at　
right　angles　to　an　inclined　surface；　and　this　would　be　their　
shortest　course　for　bringing　up　earth　from　beneath；　then　as　the　old　
burrows　collapsed　from　the　weight　of　the　superincumbent　soil；　the　
collapsing　would　inevitably　cause　the　whole　bed　of　vegetable　mould　
to　sink　or　slide　slowly　down　the　inclined　surface。　　But　to　
ascertain　the　direction　of　many　burrows　was　found　too　difficult　and　
troublesome。　　A　straight　piece　of　wire　was；　however；　pushed　into　
twenty…five　burrows　on　several　sloping　fields；　and　in　eight　cases　
the　burrows　were　nearly　at　right　angles　to　the　slope；　whilst　in　the　
remaining　cases　they　were　indifferently　directed　at　various　angles；　
either　upwards　or　downwards　with　respect　to　the　slope。

In　countries　where　the　rain　is　very　heavy；　as　in　the　tropics；　the　
castings　appear；　as　might　have　been　expected；　to　be　washed　down　in　
a　greater　degree　than　in　England。　　Mr。　Scott　informs　me　that　near　
Calcutta　the　tall　columnar　castings　（previously　described）；　the　
diameter　of　which　is　usually　between　1　and　1。5　inch；　subside　on　a　
level　surface；　after　heavy　rain；　into　almost　circular；　thin；　flat　
discs；　between　3　and　4　and　sometimes　5　inches　in　diameter。　　Three　
fresh　castings；　which　had　been　ejected　in　the　Botanic　Gardens　〃on　a　
slightly　inclined；　grass…covered；　artificial　bank　of　loamy　clay；〃　
were　carefully　measured；　and　had　a　mean　height　of　2。17；　and　a　mean　
diameter　of　1。43　inches；　these　after　heavy　rain；　formed　elongated　
patches　of　earth；　with　a　mean　length　in　the　direction　of　the　slope　
of　5。83　inches。　　As　the　earth　had　spread　very　little　up　the　slope；　
a　large　part；　judging　from　the　original　diameter　of　these　castings；　
must　have　flowed　bodily　downwards　about　4　inches。　　Moreover　some　of　
the　finest　earth　of　which　they　were　composed　must　have　been　washed　
completely　away　to　a　still　greater　distance。　　In　drier　sites　near　
Calcutta；　a　species　of　worm　ejects　its　castings；　not　in　vermiform　
masses；　but　in　little　pellets　of　varying　sizes：　　these　are　very　
numerous　in　some　places；　and　Mr。　Scott　says　that　they　〃are　washed　
away　by　every　shower。〃

I　was　led　to　believe　that　a　considerable　quantity　of　fine　earth　is　
washed　quite　away　from　castings　during　rain；　from　the　surfaces　of　
old　ones　being　often　studded　with　coarse　particles。　　Accordingly　a　
little　fine　precipitated　chalk；　moistened　with　saliva　or　gum…water；　
so　as　to　be　slightly　viscid　and　of　the　same　consistence　as　a　fresh　
casting；　was　placed　on　the　summits　of　several　castings　and　gently　
mixed　with　them。　　These　castings　were　then　watered　through　a　very　
fine　rose；　the　drops　from　which　were　closer　together　than　those　of　
rain；　but　not　nearly　so　large　as　those　in　a　thunderstorm；　nor　did　
they　strike　the　ground　with　nearly　so　much　force　as　drops　during　
heavy　rain。　　A　casting　thus　treated　subsided　with　surprising　
slowness；　owing　as　I　suppose　to　its　viscidity。　　It　did　not　flow　
bodily　down　the　grass…covered　surface　of　the　lawn；　which　was　here　
inclined　at　an　angle　of　16　degrees　20　seconds；　nevertheless　many　
particles　of　the　chalk　were　found　three　inches　below　the　casting。　　
The　experiment　was　repeated　on　three　other　castings　on　different　
parts　of　the　lawn；　which　sloped　at　2　degrees　30　seconds；　3　degrees　
and　6　degrees；　and　particles　of　chalk　could　be　seen　between　4　and　5　
inches　below　the　casting；　and　after　the　surface　had　become　dry；　
particles　were　found　in　two　cases　at　a　distance　of　5　and　6　inches。　　
Several　other　castings　with　precipitated　chalk　placed　on　their　
summits　were　left　to　the　natural　action　of　the　rain。　　In　one　case；　
after　rain　which　was　not　heavy；　the　casting　was　longitudinally　
streaked　with　white。　　In　two　other　cases　the　surface　of　the　ground　
was　rendered　somewhat　white　for　a　distance　of　one　inch　from　the　
casting；　and　some　soil　collected　at　a　distance　of　2。5　inches；　where　
the　slope　was　7　degrees；　effervesced　slightly　when　placed　in　acid。　　
After　one　or　two　weeks；　the　chalk　was　wholly　or　almost　wholly　
washed　away　from　all　the　castings　on　which　it　had　been　placed；　and　
these　had　recovered　their　natural　colour。

It　may　be　here　remarked　that　after　very　heavy　rain　shallow　pools　
may　be　seen　on　level　or　nearly　level　fields；　where　the　soil　is　not　
very　porous；　and　the　water　in　them　is　often　slightly　muddy；　when　
such　little　pools　have　dried；　the　leaves　and　blades　of　grass　at　
their　bottoms　are　generally　coated　with　a　thin　layer　of　mud。　　This　
mud　I　believe　is　derived　in　large　part　from　recently　ejected　
castings。

Dr。　King　informs　me　that　the　majority　of　the　before　described　
gigantic　casting