the formation of vegetable mould-第26章

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rather　steeply　inclined　fields　near　Down；　which　had　formerly　been　
ploughed　and　were　now　rather　sparsely　clothed　with　poor　grass；　were　
visited；　and　many　castings　extended　down　the　slopes　for　a　length　of　
5　inches；　which　was　twice　or　thrice　the　usual　diameter　of　the　
castings　thrown　up　on　the　level　parts　of　these　same　fields。　　On　
some　fine　grassy　slopes　in　Holwood　Park；　inclined　at　angles　between　
8　degrees　and　11　degrees　30　seconds　with　the　horizon；　where　the　
surface　apparently　had　never　been　disturbed　by　the　hand　of　man；　
castings　abounded　in　extraordinary　numbers：　　and　a　space　16　inches　
in　length　transversely　to　the　slope　and　6　inches　in　the　line　of　the　
slope；　was　completely　coated；　between　the　blades　of　grass；　with　a　
uniform　sheet　of　confluent　and　subsided　castings。　　Here　also　in　
many　places　the　castings　had　flowed　down　the　slope；　and　now　formed　
smooth　narrow　patches　of　earth；　6；　7；　and　7。5　inches　in　length。　　
Some　of　these　consisted　of　two　castings；　one　above　the　other；　which　
had　become　so　completely　confluent　that　they　could　hardly　be　
distinguished。　　On　my　lawn；　clothed　with　very　fine　grass；　most　of　
the　castings　are　black；　but　some　are　yellowish　from　earth　having　
been　brought　up　from　a　greater　depth　than　usual；　and　the　flowing…
down　of　these　yellow　castings　after　heavy　rain；　could　be　clearly　
seen　where　the　slope　was　5　degrees；　and　where　it　was　less　than　1　
degree　some　evidence　of　their　flowing　down　could　still　be　detected。　　
On　another　occasion；　after　rain　which　was　never　heavy；　but　which　
lasted　for　18　hours；　all　the　castings　on　this　same　gently　inclined　
lawn　had　lost　their　vermiform　structure；　and　they　had　flowed；　so　
that　fully　two…thirds　of　the　ejected　earth　lay　below　the　mouths　of　
the　burrows。

These　observations　led　me　to　make　others　with　more　care。　　Eight　
castings　were　found　on　my　lawn；　where　the　grass…blades　are　fine　and　
close　together；　and　three　others　on　a　field　with　coarse　grass。　　The　
inclination　of　the　surface　at　the　eleven　places　where　these　
castings　were　collected　varied　between　4　degrees　30　seconds　and　17　
degrees　30　seconds；　the　mean　of　the　eleven　inclinations　being　9　
degrees　26　seconds。　　The　length　of　the　castings　in　the　direction　of　
the　slope　was　first　measured　with　as　much　accuracy　as　their　
irregularities　would　permit。　　It　was　found　possible　to　make　these　
measurements　within　about　of　an　inch；　but　one　of　the　castings　was　
too　irregular　to　admit　of　measurement。　　The　average　length　in　the　
direction　of　the　slope　of　the　remaining　ten　castings　was　2。03　
inches。　　The　castings　were　then　divided　with　a　knife　into　two　parts　
along　a　horizontal　line　passing　through　the　mouth　of　the　burrow；　
which　was　discovered　by　slicing　off　the　turf；　and　all　the　ejected　
earth　was　separately　collected；　namely；　the　part　above　the　hole　and　
the　part　below。　　Afterwards　these　two　parts　were　weighed。　　In　every　
case　there　was　much　more　earth　below　than　above；　the　mean　weight　of　
that　above　being　103　grains；　and　of　that　below　205　grains；　so　that　
the　latter　was　very　nearly　double　the　former。　　As　on　level　ground　
castings　are　commonly　thrown　up　almost　equally　round　the　mouths　of　
the　burrows；　this　difference　in　weight　indicates　the　amount　of　
ejected　earth　which　had　flowed　down　the　slope。　　But　very　many　more　
observations　would　be　requisite　to　arrive　at　any　general　result；　
for　the　nature　of　the　vegetation　and　other　accidental　
circumstances；　such　as　the　heaviness　of　the　rain；　the　direction　and　
force　of　the　wind；　&c。；　appear　to　be　more　important　in　determining　
the　quantity　of　the　earth　which　flows　down　a　slope　than　its　angle。　　
Thus　with　four　castings　on　my　lawn　（included　in　the　above　eleven）　
where　the　mean　slope　was　7　degrees　19　seconds；　the　difference　in　
the　amount　of　earth　above　and　below　the　burrows　was　greater　than　
with　three　other　castings　on　the　same　lawn　where　the　mean　slope　was　
12　degrees　5　seconds。

We　may；　however；　take　the　above　eleven　cases；　which　are　accurate　as　
far　as　they　go；　and　calculate　the　weight　of　the　ejected　earth　which　
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